Many bacterial species exist simply as single cells, others associate in characteristic patterns: Neisseria form diploids (pairs), Streptococcus form chains, and Staphylococcus group together in "bunch of grapes" clusters. Bacteria can also be elongated to form filaments, for example the Actinobacteria. Filamentous bacteria are often surrounded by a sheath that contains many individual cells. Certain types, such as species of the genus Nocardia, even form complex, branched filaments, similar in appearance to fungal mycelia.


Bacteria often attach to surfaces and form dense aggregations called biofilms or bacterial mats. These films can range from a few micrometers in thickness to up to half a meter in depth, and may contain multiple species of bacteria, protists and archaea. Bacteria living in biofilms display a complex arrangement of cells and extracellular components, forming secondary structures, such as microcolonies, through which there are networks of channels to enable better diffusion of nutrients. In natural environments, such as soil or the surfaces of plants, the majority of bacteria are bound to surfaces in biofilms. Biofilms are also important in medicine, as these structures are often present during chronic bacterial infections or in infections of implanted medical devices, and bacteria protected within biofilms are much harder to kill than individual isolated bacteria.


Even more complex morphological changes are sometimes possible. For example, when starved of amino acids, Myxobacteria detect surrounding cells in a process known as quorum sensing, migrate toward each other, and aggregate to form fruiting bodies up to 500 micrometres long and containing approximately 100,000 bacterial cells. In these fruiting bodies, the bacteria perform separate tasks; this type of cooperation is a simple type of multicellular organisation. For example, about one in 10 cells migrate to the top of these fruiting bodies and differentiate into a specialised dormant state called myxospores, which are more resistant to drying and other adverse environmental conditions than are ordinary cells.


The bacterial cell is surrounded by a cell membrane (also known as a lipid, cytoplasmic or plasma membrane). This membrane encloses the contents of the cell and acts as a barrier to hold nutrients, proteins and other essential components of the cytoplasm within the cell. As they are prokaryotes, bacteria do not usually have membrane-bound organelles in their cytoplasm, and thus contain few large intracellular structures. They lack a true nucleus, mitochondria, chloroplasts and the other organelles present in eukaryotic cells. Bacteria were once seen as simple bags of cytoplasm, but structures such as the prokaryotic cytoskeleton and the localization of proteins to specific locations within the cytoplasm that give bacteria some complexity have been discovered. These subcellular levels of organization have been called "bacterial hyperstructures".


Many important biochemical reactions, such as energy generation, use concentration gradients across membranes. The general lack of internal membranes in bacteria means reactions such as electron transport occur across the cell membrane between the cytoplasm and the periplasmic space. However, in many photosynthetic bacteria the plasma membrane is highly folded and fills most of the cell with layers of light-gathering membrane. These light-gathering complexes may even form lipid-enclosed structures called chlorosomes in green sulfur bacteria. Other proteins import nutrients across the cell membrane, or expel undesired molecules from the cytoplasm.


Bacteria do not have a membrane-bound nucleus, and their genetic material is typically a single circular DNA chromosome located in the cytoplasm in an irregularly shaped body called the nucleoid. The nucleoid contains the chromosome with its associated proteins and RNA. The phylum Planctomycetes and candidate phylum Poribacteria may be exceptions to the general absence of internal membranes in bacteria, because they appear to have a double membrane around their nucleoids and contain other membrane-bound cellular structures. Like all living organisms, bacteria contain ribosomes, often grouped in chains called polyribosomes, for the production of proteins, but the structure of the bacterial ribosome is different from that of eukaryotes and Archaea. Bacterial ribosomes have a sedimentation rate of 70S (measured in Svedberg units): their subunits have rates of 30S and 50S. Some antibiotics bind specifically to 70S ribosomes and inhibit bacterial protein synthesis. Those antibiotics kill bacteria without affecting the larger 80S ribosomes of eukaryotic cells and without harming the host.


Some bacteria produce intracellular nutrient storage granules for later use, such as glycogen, polyphosphate, sulfur or polyhydroxyalkanoates. Certain bacterial species, such as the photosynthetic Cyanobacteria, produce internal gas vesicles, which they use to regulate their buoyancy – allowing them to move up or down into water layers with different light intensities and nutrient levels. Intracellular membranes called chromatophores are also found in membranes of phototrophic bacteria. Used primarily for photosynthesis, they contain bacteriochlorophyll pigments and carotenoids. An early idea was that bacteria might contain membrane folds termed mesosomes, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy. Inclusions are considered to be nonliving components of the cell that do not possess metabolic activity and are not bounded by membranes. The most common inclusions are glycogen, lipid droplets, crystals, and pigments. Volutin granules are cytoplasmic inclusions of complexed inorganic polyphosphate. These granules are called metachromatic granules due to their displaying the metachromatic effect; they appear red or blue when stained with the blue dyes methylene blue or toluidine blue. Gas vacuoles, which are freely permeable to gas, are membrane-bound vesicles present in some species of Cyanobacteria. They allow the bacteria to control their buoyancy. Microcompartments are widespread, membrane-bound organelles that are made of a protein shell that surrounds and encloses various enzymes. Carboxysomes are bacterial microcompartments that contain enzymes involved in carbon fixation. Magnetosomes are bacterial microcompartments, present in magnetotactic bacteria, that contain magnetic crystals.


In most bacteria, a cell wall is present on the outside of the cell membrane. The cell membrane and cell wall comprise the cell envelope. A common bacterial cell wall material is peptidoglycan (called "murein" in older sources), which is made from polysaccharide chains cross-linked by peptides containing D-amino acids. Bacterial cell walls are different from the cell walls of plants and fungi, which are made of cellulose and chitin, respectively. The cell wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, and the antibiotic penicillin is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan.


Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids. In contrast, gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins. Lipopolysaccharides, also called endotoxins, are composed of polysaccharides and lipid A that is responsible for much of the toxicity of gram-negative bacteria. Most bacteria have the gram-negative cell wall, and only the Firmicutes and Actinobacteria have the alternative gram-positive arrangement. These two groups were previously known as the low G+C and high G+C Gram-positive bacteria, respectively. These differences in structure can produce differences in antibiotic susceptibility; for instance, vancomycin can kill only gram-positive bacteria and is ineffective against gram-negative pathogens, such as Haemophilus influenzae or Pseudomonas aeruginosa. If the bacterial cell wall is entirely removed, it is called a protoplast, whereas if it is partially removed, it is called a spheroplast. β-Lactam antibiotics, such as penicillin, inhibit the formation of peptidoglycan cross-links in the bacterial cell wall. The enzyme lysozyme, found in human tears, also digests the cell wall of bacteria and is the body's main defense against eye infections.


Acid-fast bacteria, such as Mycobacteria, are resistant to decolorization by acids during staining procedures. The high mycolic acid content of Mycobacteria, is responsible for the staining pattern of poor absorption followed by high retention. The most common staining technique used to identify acid-fast bacteria is the Ziehl-Neelsen stain or acid-fast stain, in which the acid-fast bacilli are stained bright-red and stand out clearly against a blue background. L-form bacteria are strains of bacteria that lack cell walls. The main pathogenic bacteria in this class is Mycoplasma (not to be confused with Mycobacteria).


Fimbriae (sometimes called "attachment pili") are fine filaments of protein, usually 2–10 nanometres in diameter and up to several micrometers in length. They are distributed over the surface of the cell, and resemble fine hairs when seen under the electron microscope. Fimbriae are believed to be involved in attachment to solid surfaces or to other cells, and are essential for the virulence of some bacterial pathogens. Pili (sing. pilus) are cellular appendages, slightly larger than fimbriae, that can transfer genetic material between bacterial cells in a process called conjugation where they are called conjugation pili or "sex pili" (see bacterial genetics, below). They can also generate movement where they are called type IV pili (see movement, below).


Certain genera of Gram-positive bacteria, such as Bacillus, Clostridium, Sporohalobacter, Anaerobacter, and Heliobacterium, can form highly resistant, dormant structures called endospores. In almost all cases, one endospore is formed and this is not a reproductive process, although Anaerobacter can make up to seven endospores in a single cell. Endospores have a central core of cytoplasm containing DNA and ribosomes surrounded by a cortex layer and protected by an impermeable and rigid coat. Dipicolinic acid is a chemical compound that composes 5% to 15% of the dry weight of bacterial spores. It is implicated as responsible for the heat resistance of the endospore.


Endospores show no detectable metabolism and can survive extreme physical and chemical stresses, such as high levels of UV light, gamma radiation, detergents, disinfectants, heat, freezing, pressure, and desiccation. In this dormant state, these organisms may remain viable for millions of years, and endospores even allow bacteria to survive exposure to the vacuum and radiation in space. According to scientist Dr. Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million years old on Earth — and we know they're very hardened to radiation." Endospore-forming bacteria can also cause disease: for example, anthrax can be contracted by the inhalation of Bacillus anthracis endospores, and contamination of deep puncture wounds with Clostridium tetani endospores causes tetanus.


Bacteria exhibit an extremely wide variety of metabolic types. The distribution of metabolic traits within a group of bacteria has traditionally been used to define their taxonomy, but these traits often do not correspond with modern genetic classifications. Bacterial metabolism is classified into nutritional groups on the basis of three major criteria: the kind of energy used for growth, the source of carbon, and the electron donors used for growth. An additional criterion of respiratory microorganisms are the electron acceptors used for aerobic or anaerobic respiration.


Carbon metabolism in bacteria is either heterotrophic, where organic carbon compounds are used as carbon sources, or autotrophic, meaning that cellular carbon is obtained by fixing carbon dioxide. Heterotrophic bacteria include parasitic types. Typical autotrophic bacteria are phototrophic cyanobacteria, green sulfur-bacteria and some purple bacteria, but also many chemolithotrophic species, such as nitrifying or sulfur-oxidising bacteria. Energy metabolism of bacteria is either based on phototrophy, the use of light through photosynthesis, or based on chemotrophy, the use of chemical substances for energy, which are mostly oxidised at the expense of oxygen or alternative electron acceptors (aerobic/anaerobic respiration).


Bacteria are further divided into lithotrophs that use inorganic electron donors and organotrophs that use organic compounds as electron donors. Chemotrophic organisms use the respective electron donors for energy conservation (by aerobic/anaerobic respiration or fermentation) and biosynthetic reactions (e.g., carbon dioxide fixation), whereas phototrophic organisms use them only for biosynthetic purposes. Respiratory organisms use chemical compounds as a source of energy by taking electrons from the reduced substrate and transferring them to a terminal electron acceptor in a redox reaction. This reaction releases energy that can be used to synthesise ATP and drive metabolism. In aerobic organisms, oxygen is used as the electron acceptor. In anaerobic organisms other inorganic compounds, such as nitrate, sulfate or carbon dioxide are used as electron acceptors. This leads to the ecologically important processes of denitrification, sulfate reduction, and acetogenesis, respectively.


These processes are also important in biological responses to pollution; for example, sulfate-reducing bacteria are largely responsible for the production of the highly toxic forms of mercury (methyl- and dimethylmercury) in the environment. Non-respiratory anaerobes use fermentation to generate energy and reducing power, secreting metabolic by-products (such as ethanol in brewing) as waste. Facultative anaerobes can switch between fermentation and different terminal electron acceptors depending on the environmental conditions in which they find themselves.


Lithotrophic bacteria can use inorganic compounds as a source of energy. Common inorganic electron donors are hydrogen, carbon monoxide, ammonia (leading to nitrification), ferrous iron and other reduced metal ions, and several reduced sulfur compounds. In unusual circumstances, the gas methane can be used by methanotrophic bacteria as both a source of electrons and a substrate for carbon anabolism. In both aerobic phototrophy and chemolithotrophy, oxygen is used as a terminal electron acceptor, whereas under anaerobic conditions inorganic compounds are used instead. Most lithotrophic organisms are autotrophic, whereas organotrophic organisms are heterotrophic.


Regardless of the type of metabolic process they employ, the majority of bacteria are able to take in raw materials only in the form of relatively small molecules, which enter the cell by diffusion or through molecular channels in cell membranes. The Planctomycetes are the exception (as they are in possessing membranes around their nuclear material). It has recently been shown that Gemmata obscuriglobus is able to take in large molecules via a process that in some ways resembles endocytosis, the process used by eukaryotic cells to engulf external items.


Unlike in multicellular organisms, increases in cell size (cell growth) and reproduction by cell division are tightly linked in unicellular organisms. Bacteria grow to a fixed size and then reproduce through binary fission, a form of asexual reproduction. Under optimal conditions, bacteria can grow and divide extremely rapidly, and bacterial populations can double as quickly as every 9.8 minutes. In cell division, two identical clone daughter cells are produced. Some bacteria, while still reproducing asexually, form more complex reproductive structures that help disperse the newly formed daughter cells. Examples include fruiting body formation by Myxobacteria and aerial hyphae formation by Streptomyces, or budding. Budding involves a cell forming a protrusion that breaks away and produces a daughter cell.


In the laboratory, bacteria are usually grown using solid or liquid media. Solid growth media, such as agar plates, are used to isolate pure cultures of a bacterial strain. However, liquid growth media are used when measurement of growth or large volumes of cells are required. Growth in stirred liquid media occurs as an even cell suspension, making the cultures easy to divide and transfer, although isolating single bacteria from liquid media is difficult. The use of selective media (media with specific nutrients added or deficient, or with antibiotics added) can help identify specific organisms.


Most laboratory techniques for growing bacteria use high levels of nutrients to produce large amounts of cells cheaply and quickly. However, in natural environments, nutrients are limited, meaning that bacteria cannot continue to reproduce indefinitely. This nutrient limitation has led the evolution of different growth strategies (see r/K selection theory). Some organisms can grow extremely rapidly when nutrients become available, such as the formation of algal (and cyanobacterial) blooms that often occur in lakes during the summer. Other organisms have adaptations to harsh environments, such as the production of multiple antibiotics by Streptomyces that inhibit the growth of competing microorganisms. In nature, many organisms live in communities (e.g., biofilms) that may allow for increased supply of nutrients and protection from environmental stresses. These relationships can be essential for growth of a particular organism or group of organisms (syntrophy).


Bacterial growth follows four phases. When a population of bacteria first enter a high-nutrient environment that allows growth, the cells need to adapt to their new environment. The first phase of growth is the lag phase, a period of slow growth when the cells are adapting to the high-nutrient environment and preparing for fast growth. The lag phase has high biosynthesis rates, as proteins necessary for rapid growth are produced. The second phase of growth is the log phase, also known as the logarithmic or exponential phase. The log phase is marked by rapid exponential growth. The rate at which cells grow during this phase is known as the growth rate (k), and the time it takes the cells to double is known as the generation time (g). During log phase, nutrients are metabolised at maximum speed until one of the nutrients is depleted and starts limiting growth. The third phase of growth is the stationary phase and is caused by depleted nutrients. The cells reduce their metabolic activity and consume non-essential cellular proteins. The stationary phase is a transition from rapid growth to a stress response state and there is increased expression of genes involved in DNA repair, antioxidant metabolism and nutrient transport. The final phase is the death phase where the bacteria run out of nutrients and die.


Most bacteria have a single circular chromosome that can range in size from only 160,000 base pairs in the endosymbiotic bacteria Candidatus Carsonella ruddii, to 12,200,000 base pairs in the soil-dwelling bacteria Sorangium cellulosum. Spirochaetes of the genus Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi, the cause of Lyme disease, containing a single linear chromosome. The genes in bacterial genomes are usually a single continuous stretch of DNA and although several different types of introns do exist in bacteria, these are much rarer than in eukaryotes.


Bacteria, as asexual organisms, inherit identical copies of their parent's genes (i.e., they are clonal). However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations. Mutations come from errors made during the replication of DNA or from exposure to mutagens. Mutation rates vary widely among different species of bacteria and even among different clones of a single species of bacteria. Genetic changes in bacterial genomes come from either random mutation during replication or "stress-directed mutation", where genes involved in a particular growth-limiting process have an increased mutation rate.


Transduction of bacterial genes by bacteriophage appears to be a consequence of infrequent errors during intracellular assembly of virus particles, rather than a bacterial adaptation. Conjugation, in the much-studied E. coli system is determined by plasmid genes, and is an adaptation for transferring copies of the plasmid from one bacterial host to another. It is seldom that a conjugative plasmid integrates into the host bacterial chromosome, and subsequently transfers part of the host bacterial DNA to another bacterium. Plasmid-mediated transfer of host bacterial DNA also appears to be an accidental process rather than a bacterial adaptation.


Transformation, unlike transduction or conjugation, depends on numerous bacterial gene products that specifically interact to perform this complex process, and thus transformation is clearly a bacterial adaptation for DNA transfer. In order for a bacterium to bind, take up and recombine donor DNA into its own chromosome, it must first enter a special physiological state termed competence (see Natural competence). In Bacillus subtilis, about 40 genes are required for the development of competence. The length of DNA transferred during B. subtilis transformation can be between a third of a chromosome up to the whole chromosome. Transformation appears to be common among bacterial species, and thus far at least 60 species are known to have the natural ability to become competent for transformation. The development of competence in nature is usually associated with stressful environmental conditions, and seems to be an adaptation for facilitating repair of DNA damage in recipient cells.


In ordinary circumstances, transduction, conjugation, and transformation involve transfer of DNA between individual bacteria of the same species, but occasionally transfer may occur between individuals of different bacterial species and this may have significant consequences, such as the transfer of antibiotic resistance. In such cases, gene acquisition from other bacteria or the environment is called horizontal gene transfer and may be common under natural conditions. Gene transfer is particularly important in antibiotic resistance as it allows the rapid transfer of resistance genes between different pathogens.


Bacteriophages are viruses that infect bacteria. Many types of bacteriophage exist, some simply infect and lyse their host bacteria, while others insert into the bacterial chromosome. A bacteriophage can contain genes that contribute to its host's phenotype: for example, in the evolution of Escherichia coli O157:H7 and Clostridium botulinum, the toxin genes in an integrated phage converted a harmless ancestral bacterium into a lethal pathogen. Bacteria resist phage infection through restriction modification systems that degrade foreign DNA, and a system that uses CRISPR sequences to retain fragments of the genomes of phage that the bacteria have come into contact with in the past, which allows them to block virus replication through a form of RNA interference. This CRISPR system provides bacteria with acquired immunity to infection.


Bacterial species differ in the number and arrangement of flagella on their surface; some have a single flagellum (monotrichous), a flagellum at each end (amphitrichous), clusters of flagella at the poles of the cell (lophotrichous), while others have flagella distributed over the entire surface of the cell (peritrichous). The bacterial flagella is the best-understood motility structure in any organism and is made of about 20 proteins, with approximately another 30 proteins required for its regulation and assembly. The flagellum is a rotating structure driven by a reversible motor at the base that uses the electrochemical gradient across the membrane for power. This motor drives the motion of the filament, which acts as a propeller.


Classification seeks to describe the diversity of bacterial species by naming and grouping organisms based on similarities. Bacteria can be classified on the basis of cell structure, cellular metabolism or on differences in cell components, such as DNA, fatty acids, pigments, antigens and quinones. While these schemes allowed the identification and classification of bacterial strains, it was unclear whether these differences represented variation between distinct species or between strains of the same species. This uncertainty was due to the lack of distinctive structures in most bacteria, as well as lateral gene transfer between unrelated species. Due to lateral gene transfer, some closely related bacteria can have very different morphologies and metabolisms. To overcome this uncertainty, modern bacterial classification emphasizes molecular systematics, using genetic techniques such as guanine cytosine ratio determination, genome-genome hybridization, as well as sequencing genes that have not undergone extensive lateral gene transfer, such as the rRNA gene. Classification of bacteria is determined by publication in the International Journal of Systematic Bacteriology, and Bergey's Manual of Systematic Bacteriology. The International Committee on Systematic Bacteriology (ICSB) maintains international rules for the naming of bacteria and taxonomic categories and for the ranking of them in the International Code of Nomenclature of Bacteria.


The term "bacteria" was traditionally applied to all microscopic, single-cell prokaryotes. However, molecular systematics showed prokaryotic life to consist of two separate domains, originally called Eubacteria and Archaebacteria, but now called Bacteria and Archaea that evolved independently from an ancient common ancestor. The archaea and eukaryotes are more closely related to each other than either is to the bacteria. These two domains, along with Eukarya, are the basis of the three-domain system, which is currently the most widely used classification system in microbiolology. However, due to the relatively recent introduction of molecular systematics and a rapid increase in the number of genome sequences that are available, bacterial classification remains a changing and expanding field. For example, a few biologists argue that the Archaea and Eukaryotes evolved from Gram-positive bacteria.


The Gram stain, developed in 1884 by Hans Christian Gram, characterises bacteria based on the structural characteristics of their cell walls. The thick layers of peptidoglycan in the "Gram-positive" cell wall stain purple, while the thin "Gram-negative" cell wall appears pink. By combining morphology and Gram-staining, most bacteria can be classified as belonging to one of four groups (Gram-positive cocci, Gram-positive bacilli, Gram-negative cocci and Gram-negative bacilli). Some organisms are best identified by stains other than the Gram stain, particularly mycobacteria or Nocardia, which show acid-fastness on Ziehl–Neelsen or similar stains. Other organisms may need to be identified by their growth in special media, or by other techniques, such as serology.


As with bacterial classification, identification of bacteria is increasingly using molecular methods. Diagnostics using DNA-based tools, such as polymerase chain reaction, are increasingly popular due to their specificity and speed, compared to culture-based methods. These methods also allow the detection and identification of "viable but nonculturable" cells that are metabolically active but non-dividing. However, even using these improved methods, the total number of bacterial species is not known and cannot even be estimated with any certainty. Following present classification, there are a little less than 9,300 known species of prokaryotes, which includes bacteria and archaea; but attempts to estimate the true number of bacterial diversity have ranged from 107 to 109 total species – and even these diverse estimates may be off by many orders of magnitude.


Some species of bacteria kill and then consume other microorganisms, these species are called predatory bacteria. These include organisms such as Myxococcus xanthus, which forms swarms of cells that kill and digest any bacteria they encounter. Other bacterial predators either attach to their prey in order to digest them and absorb nutrients, such as Vampirovibrio chlorellavorus, or invade another cell and multiply inside the cytosol, such as Daptobacter. These predatory bacteria are thought to have evolved from saprophages that consumed dead microorganisms, through adaptations that allowed them to entrap and kill other organisms.


Certain bacteria form close spatial associations that are essential for their survival. One such mutualistic association, called interspecies hydrogen transfer, occurs between clusters of anaerobic bacteria that consume organic acids, such as butyric acid or propionic acid, and produce hydrogen, and methanogenic Archaea that consume hydrogen. The bacteria in this association are unable to consume the organic acids as this reaction produces hydrogen that accumulates in their surroundings. Only the intimate association with the hydrogen-consuming Archaea keeps the hydrogen concentration low enough to allow the bacteria to grow.


In soil, microorganisms that reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) carry out nitrogen fixation, converting nitrogen gas to nitrogenous compounds. This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of over 1,000 bacterial species in the normal human gut flora of the intestines can contribute to gut immunity, synthesise vitamins, such as folic acid, vitamin K and biotin, convert sugars to lactic acid (see Lactobacillus), as well as fermenting complex undigestible carbohydrates. The presence of this gut flora also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion) and these beneficial bacteria are consequently sold as probiotic dietary supplements.


If bacteria form a parasitic association with other organisms, they are classed as pathogens. Pathogenic bacteria are a major cause of human death and disease and cause infections such as tetanus, typhoid fever, diphtheria, syphilis, cholera, foodborne illness, leprosy and tuberculosis. A pathogenic cause for a known medical disease may only be discovered many years after, as was the case with Helicobacter pylori and peptic ulcer disease. Bacterial diseases are also important in agriculture, with bacteria causing leaf spot, fire blight and wilts in plants, as well as Johne's disease, mastitis, salmonella and anthrax in farm animals.


Each species of pathogen has a characteristic spectrum of interactions with its human hosts. Some organisms, such as Staphylococcus or Streptococcus, can cause skin infections, pneumonia, meningitis and even overwhelming sepsis, a systemic inflammatory response producing shock, massive vasodilation and death. Yet these organisms are also part of the normal human flora and usually exist on the skin or in the nose without causing any disease at all. Other organisms invariably cause disease in humans, such as the Rickettsia, which are obligate intracellular parasites able to grow and reproduce only within the cells of other organisms. One species of Rickettsia causes typhus, while another causes Rocky Mountain spotted fever. Chlamydia, another phylum of obligate intracellular parasites, contains species that can cause pneumonia, or urinary tract infection and may be involved in coronary heart disease. Finally, some species, such as Pseudomonas aeruginosa, Burkholderia cenocepacia, and Mycobacterium avium, are opportunistic pathogens and cause disease mainly in people suffering from immunosuppression or cystic fibrosis.


Bacterial infections may be treated with antibiotics, which are classified as bacteriocidal if they kill bacteria, or bacteriostatic if they just prevent bacterial growth. There are many types of antibiotics and each class inhibits a process that is different in the pathogen from that found in the host. An example of how antibiotics produce selective toxicity are chloramphenicol and puromycin, which inhibit the bacterial ribosome, but not the structurally different eukaryotic ribosome. Antibiotics are used both in treating human disease and in intensive farming to promote animal growth, where they may be contributing to the rapid development of antibiotic resistance in bacterial populations. Infections can be prevented by antiseptic measures such as sterilizing the skin prior to piercing it with the needle of a syringe, and by proper care of indwelling catheters. Surgical and dental instruments are also sterilized to prevent contamination by bacteria. Disinfectants such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection.


The ability of bacteria to degrade a variety of organic compounds is remarkable and has been used in waste processing and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up oil spills. Fertilizer was added to some of the beaches in Prince William Sound in an attempt to promote the growth of these naturally occurring bacteria after the 1989 Exxon Valdez oil spill. These efforts were effective on beaches that were not too thickly covered in oil. Bacteria are also used for the bioremediation of industrial toxic wastes. In the chemical industry, bacteria are most important in the production of enantiomerically pure chemicals for use as pharmaceuticals or agrichemicals.


Because of their ability to quickly grow and the relative ease with which they can be manipulated, bacteria are the workhorses for the fields of molecular biology, genetics and biochemistry. By making mutations in bacterial DNA and examining the resulting phenotypes, scientists can determine the function of genes, enzymes and metabolic pathways in bacteria, then apply this knowledge to more complex organisms. This aim of understanding the biochemistry of a cell reaches its most complex expression in the synthesis of huge amounts of enzyme kinetic and gene expression data into mathematical models of entire organisms. This is achievable in some well-studied bacteria, with models of Escherichia coli metabolism now being produced and tested. This understanding of bacterial metabolism and genetics allows the use of biotechnology to bioengineer bacteria for the production of therapeutic proteins, such as insulin, growth factors, or antibodies.


Bacteria were first observed by the Dutch microscopist Antonie van Leeuwenhoek in 1676, using a single-lens microscope of his own design. He then published his observations in a series of letters to the Royal Society of London. Bacteria were Leeuwenhoek's most remarkable microscopic discovery. They were just at the limit of what his simple lenses could make out and, in one of the most striking hiatuses in the history of science, no one else would see them again for over a century. Only then were his by-then-largely-forgotten observations of bacteria — as opposed to his famous "animalcules" (spermatozoa) — taken seriously.


Though it was known in the nineteenth century that bacteria are the cause of many diseases, no effective antibacterial treatments were available. In 1910, Paul Ehrlich developed the first antibiotic, by changing dyes that selectively stained Treponema pallidum — the spirochaete that causes syphilis — into compounds that selectively killed the pathogen. Ehrlich had been awarded a 1908 Nobel Prize for his work on immunology, and pioneered the use of stains to detect and identify bacteria, with his work being the basis of the Gram stain and the Ziehl–Neelsen stain.


When the board has no embedded components it is more correctly called a printed wiring board (PWB) or etched wiring board. However, the term printed wiring board has fallen into disuse. A PCB populated with electronic components is called a printed circuit assembly (PCA), printed circuit board assembly or PCB assembly (PCBA). The IPC preferred term for assembled boards is circuit card assembly (CCA), and for assembled backplanes it is backplane assemblies. The term PCB is used informally both for bare and assembled boards.


Initially PCBs were designed manually by creating a photomask on a clear mylar sheet, usually at two or four times the true size. Starting from the schematic diagram the component pin pads were laid out on the mylar and then traces were routed to connect the pads. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout. To fabricate the board, the finished photomask was photolithographically reproduced onto a photoresist coating on the blank copper-clad boards.


Panelization is a procedure whereby a number of PCBs are grouped for manufacturing onto a larger board - the panel. Usually a panel consists of a single design but sometimes multiple designs are mixed on a single panel. There are two types of panels: assembly panels - often called arrays - and bare board manufacturing panels. The assemblers often mount components on panels rather than single PCBs because this is efficient. The bare board manufactures always uses panels, not only for efficiency, but because of the requirements the plating process. Thus a manufacturing panel can consist of a grouping of individual PCBs or of arrays, depending on what must be delivered.


The panel is eventually broken apart into individual PCBs; this is called depaneling. Separating the individual PCBs is frequently aided by drilling or routing perforations along the boundaries of the individual circuits, much like a sheet of postage stamps. Another method, which takes less space, is to cut V-shaped grooves across the full dimension of the panel. The individual PCBs can then be broken apart along this line of weakness. Today depaneling is often done by lasers which cut the board with no contact. Laser panelization reduces stress on the fragile circuits.


Subtractive methods remove copper from an entirely copper-coated board to leave only the desired copper pattern. In additive methods the pattern is electroplated onto a bare substrate using a complex process. The advantage of the additive method is that less material is needed and less waste is produced. In the full additive process the bare laminate is covered with a photosensitive film which is imaged (exposed to light through a mask and then developed which removes the unexposed film). The exposed areas are sensitized in a chemical bath, usually containing palladium and similar to that used for through hole plating which makes the exposed area capable of bonding metal ions. The laminate is then plated with copper in the sensitized areas. When the mask is stripped, the PCB is finished.


Semi-additive is the most common process: The unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed bare original copper laminate from the board, isolating the individual traces. Some single-sided boards which have plated-through holes are made in this way. General Electric made consumer radio sets in the late 1960s using additive boards.


The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in which the board is submerged in etching solution such as ferric chloride. Compared with methods used for mass production, the etching time is long. Heat and agitation can be applied to the bath to speed the etching rate. In bubble etching, air is passed through the etchant bath to agitate the solution and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the process has become commercially obsolete since it is not as fast as spray etching. In spray etching, the etchant solution is distributed over the boards by nozzles, and recirculated by pumps. Adjustment of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of etching rates and high production rates.


Multi-layer printed circuit boards have trace layers inside the board. This is achieved by laminating a stack of materials in a press by applying pressure and heat for a period of time. This results in an inseparable one piece product. For example, a four-layer PCB can be fabricated by starting from a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom pre-preg and copper foil. It is then drilled, plated, and etched again to get traces on top and bottom layers.


Holes through a PCB are typically drilled with small-diameter drill bits made of solid coated tungsten carbide. Coated tungsten carbide is recommended since many board materials are very abrasive and drilling must be high RPM and high feed to be cost effective. Drill bits must also remain sharp so as not to mar or tear the traces. Drilling with high-speed-steel is simply not feasible since the drill bits will dull quickly and thus tear the copper and ruin the boards. The drilling is performed by automated drilling machines with placement controlled by a drill tape or drill file. These computer-generated files are also called numerically controlled drill (NCD) files or "Excellon files". The drill file describes the location and size of each drilled hole.


The hole walls for boards with two or more layers can be made conductive and then electroplated with copper to form plated-through holes.  These holes electrically connect the conducting layers of the PCB. For multi-layer boards, those with three layers or more, drilling typically produces a smear of the high temperature decomposition products of bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch. The de-smear process ensures that a good connection is made to the copper layers when the hole is plated through. On high reliability boards a process called etch-back is performed chemically with a potassium permanganate based etchant or plasma. The etch-back removes resin and the glass fibers so that the copper layers extend into the hole and as the hole is plated become integral with the deposited copper.


Matte solder is usually fused to provide a better bonding surface or stripped to bare copper. Treatments, such as benzimidazolethiol, prevent surface oxidation of bare copper. The places to which components will be mounted are typically plated, because untreated bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was coated with solder by hot air solder levelling (HASL). The HASL finish prevents oxidation from the underlying copper, thereby guaranteeing a solderable surface. This solder was a tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU and US, which restricts the use of lead. One of these lead-free compounds is SN100CL, made up of 99.3% tin, 0.7% copper, 0.05% nickel, and a nominal of 60ppm germanium.


Other platings used are OSP (organic surface protectant), immersion silver (IAg), immersion tin, electroless nickel with immersion gold coating (ENIG), electroless nickel electroless palladium immersion gold (ENEPIG) and direct gold plating (over nickel). Edge connectors, placed along one edge of some boards, are often nickel plated then gold plated. Another coating consideration is rapid diffusion of coating metal into Tin solder. Tin forms intermetallics such as Cu5Sn6 and Ag3Cu that dissolve into the Tin liquidus or solidus(@50C), stripping surface coating or leaving voids.


Electrochemical migration (ECM) is the growth of conductive metal filaments on or in a printed circuit board (PCB) under the influence of a DC voltage bias. Silver, zinc, and aluminum are known to grow whiskers under the influence of an electric field. Silver also grows conducting surface paths in the presence of halide and other ions, making it a poor choice for electronics use. Tin will grow "whiskers" due to tension in the plated surface. Tin-Lead or solder plating also grows whiskers, only reduced by the percentage Tin replaced. Reflow to melt solder or tin plate to relieve surface stress lowers whisker incidence. Another coating issue is tin pest, the transformation of tin to a powdery allotrope at low temperature.


Areas that should not be soldered may be covered with solder resist (solder mask). One of the most common solder resists used today is called "LPI" (liquid photoimageable solder mask).  A photo-sensitive coating is applied to the surface of the PWB, then exposed to light through the solder mask image film, and finally developed where the unexposed areas are washed away. Dry film solder mask is similar to the dry film used to image the PWB for plating or etching. After being laminated to the PWB surface it is imaged and develop as LPI. Once common but no longer commonly used because of its low accuracy and resolution is to screen print epoxy ink. Solder resist also provides protection from the environment.


Unpopulated boards are usually bare-board tested for "shorts" and "opens". A short is a connection between two points that should not be connected. An open is a missing connection between points that should be connected. For high-volume production a fixture or a rigid needle adapter is used to make contact with copper lands on the board. Building the adapter is a significant fixed cost and is only economical for high-volume or high-value production. For small or medium volume production flying probe testers are used where test probes are moved over the board by an XY drive to make contact with the copper lands. The CAM system instructs the electrical tester to apply a voltage to each contact point as required and to check that this voltage appears on the appropriate contact points and only on these.


Often, through-hole and surface-mount construction must be combined in a single assembly because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress, while components that are expected to go untouched will take up less space using surface-mount techniques. For further comparison, see the SMT page.


In boundary scan testing, test circuits integrated into various ICs on the board form temporary connections between the PCB traces to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be tested use a standard test configuration procedure, the most common one being the Joint Test Action Group (JTAG) standard. The JTAG test architecture provides a means to test interconnects between integrated circuits on a board without using physical test probes. JTAG tool vendors provide various types of stimulus and sophisticated algorithms, not only to detect the failing nets, but also to isolate the faults to specific nets, devices, and pins.


PCBs intended for extreme environments often have a conformal coating, which is applied by dipping or spraying after the components have been soldered. The coat prevents corrosion and leakage currents or shorting due to condensation. The earliest conformal coats were wax; modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Another technique for applying a conformal coating is for plastic to be sputtered onto the PCB in a vacuum chamber. The chief disadvantage of conformal coatings is that servicing of the board is rendered extremely difficult.


Many assembled PCBs are static sensitive, and therefore must be placed in antistatic bags during transport. When handling these boards, the user must be grounded (earthed). Improper handling techniques might transmit an accumulated static charge through the board, damaging or destroying components. Even bare boards are sometimes static sensitive. Traces have become so fine that it's quite possible to blow an etch off the board (or change its characteristics) with a static charge. This is especially true on non-traditional PCBs such as MCMs and microwave PCBs.


The first PCBs used through-hole technology, mounting electronic components by leads inserted through holes on one side of the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or more compact double-sided boards, with components soldered on both sides. Horizontal installation of through-hole parts with two axial leads (such as resistors, capacitors, and diodes) is done by bending the leads 90 degrees in the same direction, inserting the part in the board (often bending leads located on the back of the board in opposite directions to improve the part's mechanical strength), soldering the leads, and trimming off the ends. Leads may be soldered either manually or by a wave soldering machine.


Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and limits the available routing area for signal traces on layers immediately below the top layer on multi-layer boards since the holes must pass through all layers to the opposite side. Once surface-mounting came into use, small-sized SMD components were used where possible, with through-hole mounting only of components unsuitably large for surface-mounting due to power requirements or mechanical limitations, or subject to mechanical stress which might damage the PCB.


Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became widely used by the mid-1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor costs and greatly increasing production rates. Components can be supplied mounted on carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper; prices of semiconductor surface mount devices (SMDs) are determined more by the chip itself than the package, with little price advantage over larger packages. Some wire-ended components, such as 1N4148 small-signal switch diodes, are actually significantly cheaper than SMD equivalents.


Each trace consists of a flat, narrow part of the copper foil that remains after etching. The resistance, determined by width and thickness, of the traces must be sufficiently low for the current the conductor will carry. Power and ground traces may need to be wider than signal traces. In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane for shielding and power return. For microwave circuits, transmission lines can be laid out in the form of stripline and microstrip with carefully controlled dimensions to assure a consistent impedance. In radio-frequency and fast switching circuits the inductance and capacitance of the printed circuit board conductors become significant circuit elements, usually undesired; but they can be used as a deliberate part of the circuit design, obviating the need for additional discrete components.


The cloth or fiber material used, resin material, and the cloth to resin ratio determine the laminate's type designation (FR-4, CEM-1, G-10, etc.) and therefore the characteristics of the laminate produced. Important characteristics are the level to which the laminate is fire retardant, the dielectric constant (er), the loss factor (tδ), the tensile strength, the shear strength, the glass transition temperature (Tg), and the Z-axis expansion coefficient (how much the thickness changes with temperature).


There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well known pre-preg materials used in the PCB industry are FR-2 (phenolic cotton paper), FR-3 (cotton paper and epoxy), FR-4 (woven glass and epoxy), FR-5 (woven glass and epoxy), FR-6 (matte glass and polyester), G-10 (woven glass and epoxy), CEM-1 (cotton paper and epoxy), CEM-2 (cotton paper and epoxy), CEM-3 (non-woven glass and epoxy), CEM-4 (woven glass and epoxy), CEM-5 (woven glass and polyester). Thermal expansion is an important consideration especially with ball grid array (BGA) and naked die technologies, and glass fiber offers the best dimensional stability.


The reinforcement type defines two major classes of materials - woven and non-woven. Woven reinforcements are cheaper, but the high dielectric constant of glass may not be favorable for many higher-frequency applications. The spatially nonhomogeneous structure also introduces local variations in electrical parameters, due to different resin/glass ratio at different areas of the weave pattern. Nonwoven reinforcements, or materials with low or no reinforcement, are more expensive but more suitable for some RF/analog applications.


At the glass transition temperature the resin in the composite softens and significantly increases thermal expansion; exceeding Tg then exerts mechanical overload on the board components - e.g. the joints and the vias. Below Tg the thermal expansion of the resin roughly matches copper and glass, above it gets significantly higher. As the reinforcement and copper confine the board along the plane, virtually all volume expansion projects to the thickness and stresses the plated-through holes. Repeated soldering or other exposition to higher temperatures can cause failure of the plating, especially with thicker boards; thick boards therefore require high Tg matrix.


Moisture absorption occurs when the material is exposed to high humidity or water. Both the resin and the reinforcement may absorb water; water may be also soaked by capillary forces through voids in the materials and along the reinforcement. Epoxies of the FR-4 materials aren't too susceptible, with absorption of only 0.15%. Teflon has very low absorption of 0.01%. Polyimides and cyanate esters, on the other side, suffer from high water absorption. Absorbed water can lead to significant degradation of key parameters; it impairs tracking resistance, breakdown voltage, and dielectric parameters. Relative dielectric constant of water is about 73, compared to about 4 for common circuitboard materials. Absorbed moisture can also vaporize on heating and cause cracking and delamination, the same effect responsible for "popcorning" damage on wet packaging of electronic parts. Careful baking of the substrates may be required.


The printed circuit board industry defines heavy copper as layers exceeding three ounces of copper, or approximately 0.0042 inches (4.2 mils, 105 μm) thick. PCB designers and fabricators often use heavy copper when design and manufacturing circuit boards in order to increase current-carrying capacity as well as resistance to thermal strains. Heavy copper plated vias transfer heat to external heat sinks. IPC 2152 is a standard for determining current-carrying capacity of printed circuit board traces.


Since it was quite easy to stack interconnections (wires) inside the embedding matrix, the approach allowed designers to forget completely about the routing of wires (usually a time-consuming operation of PCB design): Anywhere the designer needs a connection, the machine will draw a wire in straight line from one location/pin to another. This led to very short design times (no complex algorithms to use even for high density designs) as well as reduced crosstalk (which is worse when wires run parallel to each other—which almost never happens in Multiwire), though the cost is too high to compete with cheaper PCB technologies when large quantities are needed.


Cordwood construction can save significant space and was often used with wire-ended components in applications where space was at a premium (such as missile guidance and telemetry systems) and in high-speed computers, where short traces were important. In cordwood construction, axial-leaded components were mounted between two parallel planes. The components were either soldered together with jumper wire, or they were connected to other components by thin nickel ribbon welded at right angles onto the component leads. To avoid shorting together different interconnection layers, thin insulating cards were placed between them. Perforations or holes in the cards allowed component leads to project through to the next interconnection layer. One disadvantage of this system was that special nickel-leaded components had to be used to allow the interconnecting welds to be made. Differential thermal expansion of the component could put pressure on the leads of the components and the PCB traces and cause physical damage (as was seen in several modules on the Apollo program). Additionally, components located in the interior are difficult to replace. Some versions of cordwood construction used soldered single-sided PCBs as the interconnection method (as pictured), allowing the use of normal-leaded components.


Development of the methods used in modern printed circuit boards started early in the 20th century. In 1903, a German inventor, Albert Hanson, described flat foil conductors laminated to an insulating board, in multiple layers. Thomas Edison experimented with chemical methods of plating conductors onto linen paper in 1904. Arthur Berry in 1913 patented a print-and-etch method in Britain, and in the United States Max Schoop obtained a patent to flame-spray metal onto a board through a patterned mask. Charles Ducas in 1927 patented a method of electroplating circuit patterns.


The Austrian engineer Paul Eisler invented the printed circuit as part of a radio set while working in England around 1936. Around 1943 the USA began to use the technology on a large scale to make proximity fuses for use in World War II. After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army. At around the same time in Britain work along similar lines was carried out by Geoffrey Dummer, then at the RRDE.


During World War II, the development of the anti-aircraft proximity fuse required an electronic circuit that could withstand being fired from a gun, and could be produced in quantity. The Centralab Division of Globe Union submitted a proposal which met the requirements: a ceramic plate would be screenprinted with metallic paint for conductors and carbon material for resistors, with ceramic disc capacitors and subminiature vacuum tubes soldered in place. The technique proved viable, and the resulting patent on the process, which was classified by the U.S. Army, was assigned to Globe Union. It was not until 1984 that the Institute of Electrical and Electronics Engineers (IEEE) awarded Mr. Harry W. Rubinstein, the former head of Globe Union's Centralab Division, its coveted Cledo Brunetti Award for early key contributions to the development of printed components and conductors on a common insulating substrate. As well, Mr. Rubinstein was honored in 1984 by his alma mater, the University of Wisconsin-Madison, for his innovations in the technology of printed electronic circuits and the fabrication of capacitors.


Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components' leads were then passed through the holes and soldered to the PCB trace. This method of assembly is called through-hole construction. In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered. The patent they obtained in 1956 was assigned to the U.S. Army. With the development of board lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are wasteful since drilling holes is expensive and the protruding wires are merely cut off.


Greek colonies and communities have been historically established on the shores of the Mediterranean Sea and Black Sea, but the Greek people have always been centered around the Aegean and Ionian seas, where the Greek language has been spoken since the Bronze Age. Until the early 20th century, Greeks were distributed between the Greek peninsula, the western coast of Asia Minor, the Black Sea coast, Cappadocia in central Anatolia, Egypt, the Balkans, Cyprus, and Constantinople. Many of these regions coincided to a large extent with the borders of the Byzantine Empire of the late 11th century and the Eastern Mediterranean areas of ancient Greek colonization. The cultural centers of the Greeks have included Athens, Thessalonica, Alexandria, Smyrna, and Constantinople at various periods.


The evolution of Proto-Greek should be considered within the context of an early Paleo-Balkan sprachbund that makes it difficult to delineate exact boundaries between individual languages. The characteristically Greek representation of word-initial laryngeals by prothetic vowels is shared, for one, by the Armenian language, which also seems to share some other phonological and morphological peculiarities of Greek; this has led some linguists to propose a hypothetical closer relationship between Greek and Armenian, although evidence remains scant.


Around 1200 BC, the Dorians, another Greek-speaking people, followed from Epirus. Traditionally, historians have believed that the Dorian invasion caused the collapse of the Mycenaean civilization, but it is likely the main attack was made by seafaring raiders (sea peoples) who sailed into the eastern Mediterranean around 1180 BC. The Dorian invasion was followed by a poorly attested period of migrations, appropriately called the Greek Dark Ages, but by 800 BC the landscape of Archaic and Classical Greece was discernible.


The Greeks of classical antiquity idealized their Mycenaean ancestors and the Mycenaean period as a glorious era of heroes, closeness of the gods and material wealth. The Homeric Epics (i.e. Iliad and Odyssey) were especially and generally accepted as part of the Greek past and it was not until the 19th century that scholars began to question Homer's historicity. As part of the Mycenaean heritage that survived, the names of the gods and goddesses of Mycenaean Greece (e.g. Zeus, Poseidon and Hades) became major figures of the Olympian Pantheon of later antiquity.


The ethnogenesis of the Greek nation is linked to the development of Pan-Hellenism in the 8th century BC. According to some scholars, the foundational event was the Olympic Games in 776 BC, when the idea of a common Hellenism among the Greek tribes was first translated into a shared cultural experience and Hellenism was primarily a matter of common culture. The works of Homer (i.e. Iliad and Odyssey) and Hesiod (i.e. Theogony) were written in the 8th century BC, becoming the basis of the national religion, ethos, history and mythology. The Oracle of Apollo at Delphi was established in this period.


The classical period of Greek civilization covers a time spanning from the early 5th century BC to the death of Alexander the Great, in 323 BC (some authors prefer to split this period into 'Classical', from the end of the Persian wars to the end of the Peloponnesian War, and 'Fourth Century', up to the death of Alexander). It is so named because it set the standards by which Greek civilization would be judged in later eras. The Classical period is also described as the "Golden Age" of Greek civilization, and its art, philosophy, architecture and literature would be instrumental in the formation and development of Western culture.


In any case, Alexander's toppling of the Achaemenid Empire, after his victories at the battles of the Granicus, Issus and Gaugamela, and his advance as far as modern-day Pakistan and Tajikistan, provided an important outlet for Greek culture, via the creation of colonies and trade routes along the way. While the Alexandrian empire did not survive its creator's death intact, the cultural implications of the spread of Hellenism across much of the Middle East and Asia were to prove long lived as Greek became the lingua franca, a position it retained even in Roman times. Many Greeks settled in Hellenistic cities like Alexandria, Antioch and Seleucia. Two thousand years later, there are still communities in Pakistan and Afghanistan, like the Kalash, who claim to be descended from Greek settlers.


This age saw the Greeks move towards larger cities and a reduction in the importance of the city-state. These larger cities were parts of the still larger Kingdoms of the Diadochi. Greeks, however, remained aware of their past, chiefly through the study of the works of Homer and the classical authors. An important factor in maintaining Greek identity was contact with barbarian (non-Greek) peoples, which was deepened in the new cosmopolitan environment of the multi-ethnic Hellenistic kingdoms. This led to a strong desire among Greeks to organize the transmission of the Hellenic paideia to the next generation. Greek science, technology and mathematics are generally considered to have reached their peak during the Hellenistic period.


In the religious sphere, this was a period of profound change. The spiritual revolution that took place, saw a waning of the old Greek religion, whose decline beginning in the 3rd century BC continued with the introduction of new religious movements from the East. The cults of deities like Isis and Mithra were introduced into the Greek world. Greek-speaking communities of the Hellenized East were instrumental in the spread of early Christianity in the 2nd and 3rd centuries, and Christianity's early leaders and writers (notably St Paul) were generally Greek-speaking, though none were from Greece. However, Greece itself had a tendency to cling to paganism and was not one of the influential centers of early Christianity: in fact, some ancient Greek religious practices remained in vogue until the end of the 4th century, with some areas such as the southeastern Peloponnese remaining pagan until well into the 10th century AD.


Of the new eastern religions introduced into the Greek world, the most successful was Christianity. From the early centuries of the Common Era, the Greeks identified as Romaioi ("Romans"), by that time the name ‘Hellenes’ denoted pagans. While ethnic distinctions still existed in the Roman Empire, they became secondary to religious considerations and the renewed empire used Christianity as a tool to support its cohesion and promoted a robust Roman national identity. Concurrently the secular, urban civilization of late antiquity survived in the Eastern Mediterranean along with Greco-Roman educational system, although it was from Christianity that the culture's essential values were drawn.


The Eastern Roman Empire – today conventionally named the Byzantine Empire, a name not in use during its own time – became increasingly influenced by Greek culture after the 7th century, when Emperor Heraclius (AD 575 - 641) decided to make Greek the empire's official language. Certainly from then on, but likely earlier, the Roman and Greek cultures were virtually fused into a single Greco-Roman world. Although the Latin West recognized the Eastern Empire's claim to the Roman legacy for several centuries, after Pope Leo III crowned Charlemagne, king of the Franks, as the "Roman Emperor" on 25 December 800, an act which eventually led to the formation of the Holy Roman Empire, the Latin West started to favour the Franks and began to refer to the Eastern Roman Empire largely as the Empire of the Greeks (Imperium Graecorum).


A distinct Greek political identity re-emerged in the 11th century in educated circles and became more forceful after the fall of Constantinople to the Crusaders of the Fourth Crusade in 1204, so that when the empire was revived in 1261, it became in many ways a Greek national state. That new notion of nationhood engendered a deep interest in the classical past culminating in the ideas of the Neoplatonist philosopher Gemistus Pletho, who abandoned Christianity. However, it was the combination of Orthodox Christianity with a specifically Greek identity that shaped the Greeks' notion of themselves in the empire's twilight years. The interest in the Classical Greek heritage was complemented by a renewed emphasis on Greek Orthodox identity, which was reinforced in the late Medieval and Ottoman Greeks' links with their fellow Orthodox Christians in the Russian Empire. These were further strengthened following the fall of the Empire of Trebizond in 1461, after which and until the second Russo-Turkish War of 1828-29 hundreds of thousands of Pontic Greeks fled or migrated from the Pontic Alps and Armenian Highlands to southern Russia and the Russian South Caucasus (see also Greeks in Russia, Greeks in Armenia, Greeks in Georgia, and Caucasian Greeks).


Following the Fall of Constantinople on 29 May 1453, many Greeks sought better employment and education opportunities by leaving for the West, particularly Italy, Central Europe, Germany and Russia. Greeks are greatly credited for the European cultural revolution, later called, the Renaissance. In Greek-inhabited territory itself, Greeks came to play a leading role in the Ottoman Empire, due in part to the fact that the central hub of the empire, politically, culturally, and socially, was based on Western Thrace and Greek Macedonia, both in Northern Greece, and of course was centred on the mainly Greek-populated, former Byzantine capital, Constantinople. As a direct consequence of this situation, Greek-speakers came to play a hugely important role in the Ottoman trading and diplomatic establishment, as well as in the church. Added to this, in the first half of the Ottoman period men of Greek origin made up a significant proportion of the Ottoman army, navy, and state bureaucracy, having been levied as adolescents (along with especially Albanians and Serbs) into Ottoman service through the devshirme. Many Ottomans of Greek (or Albanian or Serb) origin were therefore to be found within the Ottoman forces which governed the provinces, from Ottoman Egypt, to Ottomans occupied Yemen and Algeria, frequently as provincial governors.


For those that remained under the Ottoman Empire's millet system, religion was the defining characteristic of national groups (milletler), so the exonym "Greeks" (Rumlar from the name Rhomaioi) was applied by the Ottomans to all members of the Orthodox Church, regardless of their language or ethnic origin. The Greek speakers were the only ethnic group to actually call themselves Romioi, (as opposed to being so named by others) and, at least those educated, considered their ethnicity (genos) to be Hellenic. There were, however, many Greeks who escaped the second-class status of Christians inherent in the Ottoman millet system, according to which Muslims were explicitly awarded senior status and preferential treatment. These Greeks either emigrated, particularly to their fellow Greek Orthodox protector, the Russian Empire, or simply converted to Islam, often only very superficially and whilst remaining crypto-Christian. The most notable examples of large-scale conversion to Turkish Islam among those today defined as Greek Muslims - excluding those who had to convert as a matter of course on being recruited through the devshirme - were to be found in Crete (Cretan Turks), Greek Macedonia (for example among the Vallahades of western Macedonia), and among Pontic Greeks in the Pontic Alps and Armenian Highlands. Several Ottoman sultans and princes were also of part Greek origin, with mothers who were either Greek concubines or princesses from Byzantine noble families, one famous example being sultan Selim the Grim, whose mother Gülbahar Hatun was a Pontic Greek.


The roots of Greek success in the Ottoman Empire can be traced to the Greek tradition of education and commerce. It was the wealth of the extensive merchant class that provided the material basis for the intellectual revival that was the prominent feature of Greek life in the half century and more leading to the outbreak of the Greek War of Independence in 1821. Not coincidentally, on the eve of 1821, the three most important centres of Greek learning were situated in Chios, Smyrna and Aivali, all three major centres of Greek commerce. Greek success was also favoured by Greek domination of the Christian Orthodox church.


The relationship between ethnic Greek identity and Greek Orthodox religion continued after the creation of the Modern Greek state in 1830. According to the second article of the first Greek constitution of 1822, a Greek was defined as any Christian resident of the Kingdom of Greece, a clause removed by 1840. A century later, when the Treaty of Lausanne was signed between Greece and Turkey in 1923, the two countries agreed to use religion as the determinant for ethnic identity for the purposes of population exchange, although most of the Greeks displaced (over a million of the total 1.5 million) had already been driven out by the time the agreement was signed.[note 1] The Greek genocide, in particular the harsh removal of Pontian Greeks from the southern shore area of the Black Sea, contemporaneous with and following the failed Greek Asia Minor Campaign, was part of this process of Turkification of the Ottoman Empire and the placement of its economy and trade, then largely in Greek hands under ethnic Turkish control.


The terms used to define Greekness have varied throughout history but were never limited or completely identified with membership to a Greek state. By Western standards, the term Greeks has traditionally referred to any native speakers of the Greek language, whether Mycenaean, Byzantine or modern Greek. Byzantine Greeks called themselves Romioi and considered themselves the political heirs of Rome, but at least by the 12th century a growing number of those educated, deemed themselves the heirs of ancient Greece as well, although for most of the Greek speakers, "Hellene" still meant pagan. On the eve of the Fall of Constantinople the Last Emperor urged his soldiers to remember that they were the descendants of Greeks and Romans.


Before the establishment of the Modern Greek state, the link between ancient and modern Greeks was emphasized by the scholars of Greek Enlightenment especially by Rigas Feraios. In his "Political Constitution", he addresses to the nation as "the people descendant of the Greeks". The modern Greek state was created in 1829, when the Greeks liberated a part of their historic homelands, Peloponnese, from the Ottoman Empire. The large Greek diaspora and merchant class were instrumental in transmitting the ideas of western romantic nationalism and philhellenism, which together with the conception of Hellenism, formulated during the last centuries of the Byzantine Empire, formed the basis of the Diafotismos and the current conception of Hellenism.


Homer refers to the "Hellenes" (/ˈhɛliːnz/) as a relatively small tribe settled in Thessalic Phthia, with its warriors under the command of Achilleus. The Parian Chronicle says that Phthia was the homeland of the Hellenes and that this name was given to those previously called Greeks (Γραικοί). In Greek mythology, Hellen, the patriarch of Hellenes, was son of Pyrrha and Deucalion, who ruled around Phthia, the only survivors after the great deluge. It seems that the myth was invented when the Greek tribes started to separate from each other in certain areas of Greece and it indicates their common origin. Aristotle names ancient Hellas as an area in Epirus between Dodona and the Achelous river, the location of the great deluge of Deucalion, a land occupied by the Selloi and the "Greeks" who later came to be known as "Hellenes". Selloi were the priests of Dodonian Zeus and the word probably means "sacrificers" (compare Gothic saljan, "present, sacrifice"). There is currently no satisfactory etymology of the name Hellenes. Some scholars assert that the name Selloi changed to Sellanes and then to Hellanes-Hellenes. However this etymology connects the name Hellenes with the Dorians who occupied Epirus and the relation with the name Greeks given by the Romans becomes uncertain. The name Hellenes seems to be older and it was probably used by the Greeks with the establishment of the Great Amphictyonic League. This was an ancient association of Greek tribes with twelve founders which was organized to protect the great temples of Apollo in Delphi (Phocis) and of Demeter near Thermopylae (Locris). According to the legend it was founded after the Trojan War by the eponymous Amphictyon, brother of Hellen.


In the Hesiodic Catalogue of Women, Graecus is presented as the son of Zeus and Pandora II, sister of Hellen the patriarch of Hellenes. Hellen was the son of Deucalion who ruled around Phthia in central Greece. The Parian Chronicle mentions that when Deucalion became king of Phthia, the previously called Graikoi were named Hellenes. Aristotle notes that the Hellenes were related with Grai/Greeks (Meteorologica I.xiv) a native name of a Dorian tribe in Epirus which was used by the Illyrians. He also claims that the great deluge must have occurred in the region around Dodona, where the Selloi dwelt. However, according to the Greek tradition it is more possible that the homeland of the Greeks was originally in central Greece. A modern theory derives the name Greek (Latin Graeci) from Graikos, "inhabitant of Graia/Graea," a town on the coast of Boeotia. Greek colonists from Graia helped to found Cumae (900 BC) in Italy, where they were called Graeces. When the Romans encountered them they used this name for the colonists and then for all Greeks (Graeci.) The word γραῖα graia "old woman" comes from the PIE root *ǵerh2-/*ǵreh2-, "to grow old" via Proto-Greek *gera-/grau-iu; the same root later gave γέρας geras (/keras/), "gift of honour" in Mycenean Greek. The Germanic languages borrowed the word Greeks with an initial "k" sound which probably was their initial sound closest to the Latin "g" at the time (Goth. Kreks). The area out of ancient Attica including Boeotia was called Graïke and is connected with the older deluge of Ogyges, the mythological ruler of Boeotia. The region was originally occupied by the Minyans who were autochthonous or Proto-Greek speaking people. In ancient Greek the name Ogygios came to mean "from earliest days".


Homer uses the terms Achaeans and Danaans (Δαναοί) as a generic term for Greeks in Iliad, and they were probably a part of the Mycenean civilization. The names Achaioi and Danaoi seem to be pre-Dorian belonging to the people who were overthrown. They were forced to the region that later bore the name Achaea after the Dorian invasion. In the 5th century BC, they were redefined as contemporary speakers of Aeolic Greek which was spoken mainly in Thessaly, Boeotia and Lesbos. There are many controversial theories on the origin of the Achaeans. According to one view, the Achaeans were one of the fair-headed tribes of upper Europe, who pressed down over the Alps during the early Iron age (1300 BC) to southern Europe. Another theory suggests that the Peloponnesian Dorians were the Achaeans. These theories are rejected by other scholars who, based on linguistic criteria, suggest that the Achaeans were mainland pre-Dorian Greeks. There is also the theory that there was an Achaean ethnos that migrated from Asia minor to lower Thessaly prior to 2000 BC. Some Hittite texts mention a nation lying to the west called Ahhiyava or Ahhiya. Egyptian documents refer to Ekwesh, one of the groups of sea peoples who attached Egypt during the reign of Merneptah (1213-1203 BCE), who may have been Achaeans.


In Homer's Iliad, the names Danaans (or Danaoi: Δαναοί) and Argives (Argives: Αργείοι) are used to designate the Greek forces opposed to the Trojans. The myth of Danaus, whose origin is Egypt, is a foundation legend of Argos. His daughters Danaides, were forced in Tartarus to carry a jug to fill a bathtub without a bottom. This myth is connected with a task that can never be fulfilled (Sisyphos) and the name can be derived from the PIE root *danu: "river". There is not any satisfactory theory on their origin. Some scholars connect Danaans with the Denyen, one of the groups of the sea peoples who attacked Egypt during the reign of Ramesses III (1187-1156 BCE). The same inscription mentions the Weshesh who might have been the Achaeans. The Denyen seem to have been inhabitants of the city Adana in Cilicia. Pottery similar to that of Mycenae itself has been found in Tarsus of Cilicia and it seems that some refugees from the Aegean went there after the collapse of the Mycenean civilization. These Cilicians seem to have been called Dananiyim, the same word as Danaoi who attacked Egypt in 1191 BC along with the Quaouash (or Weshesh) who may be Achaeans. They were also called Danuna according to a Hittite inscription and the same name is mentioned in the Amarna letters. Julius Pokorny reconstructs the name from the PIE root da:-: "flow, river", da:-nu: "any moving liquid, drops", da: navo "people living by the river, Skyth. nomadic people (in Rigveda water-demons, fem.Da:nu primordial goddess), in Greek Danaoi, Egypt. Danuna". It is also possible that the name Danaans is pre-Greek. A country Danaja with a city Mukana (propaply: Mycenea) is mentioned in inscriptions from Egypt from Amenophis III (1390-1352 BC), Thutmosis III (1437 BC).


The most obvious link between modern and ancient Greeks is their language, which has a documented tradition from at least the 14th century BC to the present day, albeit with a break during the Greek Dark Ages (lasting from the 11th to the 8th century BC). Scholars compare its continuity of tradition to Chinese alone. Since its inception, Hellenism was primarily a matter of common culture and the national continuity of the Greek world is a lot more certain than its demographic. Yet, Hellenism also embodied an ancestral dimension through aspects of Athenian literature that developed and influenced ideas of descent based on autochthony. During the later years of the Eastern Roman Empire, areas such as Ionia and Constantinople experienced a Hellenic revival in language, philosophy, and literature and on classical models of thought and scholarship. This revival provided a powerful impetus to the sense of cultural affinity with ancient Greece and its classical heritage. The cultural changes undergone by the Greeks are, despite a surviving common sense of ethnicity, undeniable. At the same time, the Greeks have retained their language and alphabet, certain values and cultural traditions, customs, a sense of religious and cultural difference and exclusion, (the word barbarian was used by 12th-century historian Anna Komnene to describe non-Greek speakers), a sense of Greek identity and common sense of ethnicity despite the global political and social changes of the past two millennia.


Today, Greeks are the majority ethnic group in the Hellenic Republic, where they constitute 93% of the country's population, and the Republic of Cyprus where they make up 78% of the island's population (excluding Turkish settlers in the occupied part of the country). Greek populations have not traditionally exhibited high rates of growth; nonetheless, the population of Greece has shown regular increase since the country's first census in 1828. A large percentage of the population growth since the state's foundation has resulted from annexation of new territories and the influx of 1.5 million Greek refugees after the 1923 population exchange between Greece and Turkey. About 80% of the population of Greece is urban, with 28% concentrated in the city of Athens


Greeks from Cyprus have a similar history of emigration, usually to the English-speaking world because of the island's colonization by the British Empire. Waves of emigration followed the Turkish invasion of Cyprus in 1974, while the population decreased between mid-1974 and 1977 as a result of emigration, war losses, and a temporary decline in fertility. After the ethnic cleansing of a third of the Greek population of the island in 1974, there was also an increase in the number of Greek Cypriots leaving, especially for the Middle East, which contributed to a decrease in population that tapered off in the 1990s. Today more than two-thirds of the Greek population in Cyprus is urban.


There is a sizeable Greek minority of about 105,000 (disputed, sources claim higher) people, in Albania. The Greek minority of Turkey, which numbered upwards of 200,000 people after the 1923 exchange, has now dwindled to a few thousand, after the 1955 Constantinople Pogrom and other state sponsored violence and discrimination. This effectively ended, though not entirely, the three-thousand-year-old presence of Hellenism in Asia Minor. There are smaller Greek minorities in the rest of the Balkan countries, the Levant and the Black Sea states, remnants of the Old Greek Diaspora (pre-19th century).


The total number of Greeks living outside Greece and Cyprus today is a contentious issue. Where Census figures are available, they show around 3 million Greeks outside Greece and Cyprus. Estimates provided by the SAE - World Council of Hellenes Abroad put the figure at around 7 million worldwide. According to George Prevelakis of Sorbonne University, the number is closer to just below 5 million. Integration, intermarriage, and loss of the Greek language influence the self-identification of the Omogeneia. Important centres of the New Greek Diaspora today are London, New York, Melbourne and Toronto. In 2010, the Hellenic Parliament introduced a law that enables Diaspora Greeks in Greece to vote in the elections of the Greek state. This law was later repealed in early 2014.


In ancient times, the trading and colonizing activities of the Greek tribes and city states spread the Greek culture, religion and language around the Mediterranean and Black Sea basins, especially in Sicily and southern Italy (also known as Magna Grecia), Spain, the south of France and the Black sea coasts. Under Alexander the Great's empire and successor states, Greek and Hellenizing ruling classes were established in the Middle East, India and in Egypt. The Hellenistic period is characterized by a new wave of Greek colonization that established Greek cities and kingdoms in Asia and Africa. Under the Roman Empire, easier movement of people spread Greeks across the Empire and in the eastern territories, Greek became the lingua franca rather than Latin. The modern-day Griko community of southern Italy, numbering about 60,000, may represent a living remnant of the ancient Greek populations of Italy.


During and after the Greek War of Independence, Greeks of the diaspora were important in establishing the fledgling state, raising funds and awareness abroad. Greek merchant families already had contacts in other countries and during the disturbances many set up home around the Mediterranean (notably Marseilles in France, Livorno in Italy, Alexandria in Egypt), Russia (Odessa and Saint Petersburg), and Britain (London and Liverpool) from where they traded, typically in textiles and grain. Businesses frequently comprised the extended family, and with them they brought schools teaching Greek and the Greek Orthodox Church.


Greek culture has evolved over thousands of years, with its beginning in the Mycenaean civilization, continuing through the Classical period, the Roman and Eastern Roman periods and was profoundly affected by Christianity, which it in turn influenced and shaped. Ottoman Greeks had to endure through several centuries of adversity that culminated in genocide in the 20th century but nevertheless included cultural exchanges and enriched both cultures. The Diafotismos is credited with revitalizing Greek culture and giving birth to the synthesis of ancient and medieval elements that characterize it today.


Greek demonstrates several linguistic features that are shared with other Balkan languages, such as Albanian, Bulgarian and Eastern Romance languages (see Balkan sprachbund), and has absorbed many foreign words, primarily of Western European and Turkish origin. Because of the movements of Philhellenism and the Diafotismos in the 19th century, which emphasized the modern Greeks' ancient heritage, these foreign influences were excluded from official use via the creation of Katharevousa, a somewhat artificial form of Greek purged of all foreign influence and words, as the official language of the Greek state. In 1976, however, the Hellenic Parliament voted to make the spoken Dimotiki the official language, making Katharevousa obsolete.


Modern Greek has, in addition to Standard Modern Greek or Dimotiki, a wide variety of dialects of varying levels of mutual intelligibility, including Cypriot, Pontic, Cappadocian, Griko and Tsakonian (the only surviving representative of ancient Doric Greek). Yevanic is the language of the Romaniotes, and survives in small communities in Greece, New York and Israel. In addition to Greek, many Greeks in Greece and the Diaspora are bilingual in other languages or dialects such as English, Arvanitika/Albanian, Aromanian, Macedonian Slavic, Russian and Turkish.


Most Greeks are Christians, belonging to the Greek Orthodox Church. During the first centuries after Jesus Christ, the New Testament was originally written in Koine Greek, which remains the liturgical language of the Greek Orthodox Church, and most of the early Christians and Church Fathers were Greek-speaking. There are small groups of ethnic Greeks adhering to other Christian denominations like Greek Catholics, Greek Evangelicals, Pentecostals, and groups adhering to other religions including Romaniot and Sephardic Jews and Greek Muslims. About 2,000 Greeks are members of Hellenic Polytheistic Reconstructionism congregations.


Greek art has a long and varied history. Greeks have contributed to the visual, literary and performing arts. In the West, ancient Greek art was influential in shaping the Roman and later the modern western artistic heritage. Following the Renaissance in Europe, the humanist aesthetic and the high technical standards of Greek art inspired generations of European artists. Well into the 19th century, the classical tradition derived from Greece played an important role in the art of the western world. In the East, Alexander the Great's conquests initiated several centuries of exchange between Greek, Central Asian and Indian cultures, resulting in Greco-Buddhist art, whose influence reached as far as Japan.


Notable modern Greek artists include Renaissance painter Dominikos Theotokopoulos (El Greco), Panagiotis Doxaras, Nikolaos Gyzis, Nikiphoros Lytras, Yannis Tsarouchis, Nikos Engonopoulos, Constantine Andreou, Jannis Kounellis, sculptors such as Leonidas Drosis, Georgios Bonanos, Yannoulis Chalepas and Joannis Avramidis, conductor Dimitri Mitropoulos, soprano Maria Callas, composers such as Mikis Theodorakis, Nikos Skalkottas, Iannis Xenakis, Manos Hatzidakis, Eleni Karaindrou, Yanni and Vangelis, one of the best-selling singers worldwide Nana Mouskouri and poets such as Kostis Palamas, Dionysios Solomos, Angelos Sikelianos and Yannis Ritsos. Alexandrian Constantine P. Cavafy and Nobel laureates Giorgos Seferis and Odysseas Elytis are among the most important poets of the 20th century. Novel is also represented by Alexandros Papadiamantis and Nikos Kazantzakis.


The Greeks of the Classical era made several notable contributions to science and helped lay the foundations of several western scientific traditions, like philosophy, historiography and mathematics. The scholarly tradition of the Greek academies was maintained during Roman times with several academic institutions in Constantinople, Antioch, Alexandria and other centres of Greek learning while Eastern Roman science was essentially a continuation of classical science. Greeks have a long tradition of valuing and investing in paideia (education). Paideia was one of the highest societal values in the Greek and Hellenistic world while the first European institution described as a university was founded in 5th century Constantinople and operated in various incarnations until the city's fall to the Ottomans in 1453. The University of Constantinople was Christian Europe's first secular institution of higher learning since no theological subjects were taught, and considering the original meaning of the world university as a corporation of students, the world’s first university as well.


As of 2007, Greece had the eighth highest percentage of tertiary enrollment in the world (with the percentages for female students being higher than for male) while Greeks of the Diaspora are equally active in the field of education. Hundreds of thousands of Greek students attend western universities every year while the faculty lists of leading Western universities contain a striking number of Greek names. Notable modern Greek scientists of modern times include Dimitrios Galanos, Georgios Papanikolaou (inventor of the Pap test), Nicholas Negroponte, Constantin Carathéodory, Manolis Andronikos, Michael Dertouzos, John Argyris, Panagiotis Kondylis, John Iliopoulos (2007 Dirac Prize for his contributions on the physics of the charm quark, a major contribution to the birth of the Standard Model, the modern theory of Elementary Particles), Joseph Sifakis (2007 Turing Award, the "Nobel Prize" of Computer Science), Christos Papadimitriou (2002 Knuth Prize, 2012 Gödel Prize), Mihalis Yannakakis (2005 Knuth Prize) and Dimitri Nanopoulos.


The most widely used symbol is the flag of Greece, which features nine equal horizontal stripes of blue alternating with white representing the nine syllables of the Greek national motto Eleftheria i thanatos (freedom or death), which was the motto of the Greek War of Independence. The blue square in the upper hoist-side corner bears a white cross, which represents Greek Orthodoxy. The Greek flag is widely used by the Greek Cypriots, although Cyprus has officially adopted a neutral flag to ease ethnic tensions with the Turkish Cypriot minority – see flag of Cyprus).


Greek surnames were widely in use by the 9th century supplanting the ancient tradition of using the father’s name, however Greek surnames are most commonly patronymics. Commonly, Greek male surnames end in -s, which is the common ending for Greek masculine proper nouns in the nominative case. Exceptionally, some end in -ou, indicating the genitive case of this proper noun for patronymic reasons. Although surnames in mainland Greece are static today, dynamic and changing patronymic usage survives in middle names where the genitive of father's first name is commonly the middle name (this usage having been passed on to the Russians). In Cyprus, by contrast, surnames follow the ancient tradition of being given according to the father’s name. Finally, in addition to Greek-derived surnames many have Latin, Turkish and Italian origin.


The traditional Greek homelands have been the Greek peninsula and the Aegean Sea, the Southern Italy (Magna Graecia), the Black Sea, the Ionian coasts of Asia Minor and the islands of Cyprus and Sicily. In Plato's Phaidon, Socrates remarks, "we (Greeks) live around a sea like frogs around a pond" when describing to his friends the Greek cities of the Aegean. This image is attested by the map of the Old Greek Diaspora, which corresponded to the Greek world until the creation of the Greek state in 1832. The sea and trade were natural outlets for Greeks since the Greek peninsula is rocky and does not offer good prospects for agriculture.


Notable Greek seafarers include people such as Pytheas of Marseilles, Scylax of Caryanda who sailed to Iberia and beyond, Nearchus, the 6th century merchant and later monk Cosmas Indicopleustes (Cosmas who sailed to India) and the explorer of the Northwestern passage Juan de Fuca. In later times, the Romioi plied the sea-lanes of the Mediterranean and controlled trade until an embargo imposed by the Roman Emperor on trade with the Caliphate opened the door for the later Italian pre-eminence in trade.


The Greek shipping tradition recovered during Ottoman rule when a substantial merchant middle class developed, which played an important part in the Greek War of Independence. Today, Greek shipping continues to prosper to the extent that Greece has the largest merchant fleet in the world, while many more ships under Greek ownership fly flags of convenience. The most notable shipping magnate of the 20th century was Aristotle Onassis, others being Yiannis Latsis, George Livanos, and Stavros Niarchos.


Another study from 2012 included 150 dental school students from University of Athens, the result showed that light hair colour (blonde/light ash brown) was predominant in 10.7% of the students. 36% had medium hair colour (Light brown/Medium darkest brown). 32% had darkest brown and 21% black (15.3 off black, 6% midnight black). In conclusion the hair colour of young Greeks are mostly brown, ranging from light to dark brown. with significant minorities having black and blonde hair. The same study also showed that the eye colour of the students was 14.6% blue/green, 28% medium (light brown) and 57.4% dark brown.