Klingon
The Klingons (tlhIngan in Klingonese) were a humanoid warrior species that originated from the planet Qo'noS (pronounced Kronos), an M-class planet in the Beta Quadrant. One of the major powers of the galaxy, the Klingons were a proud, tradition-bound people who valued honor and combat. The aggressive Klingon culture had made them an interstellar military power to be respected and feared.

History and politics 
Main article: Klingon history

Recorded history begins with the story of the creation of Kotar and his mate, who were recognized as the first Klingons. Together they were responsible for destroying the gods who created them. ()

Kahless the Unforgettable founded the Klingon Empire some time in the 9th century through the performance of many heroic feats. He unified the Klingon people when he killed the tyrant Molor. Kahless came to be revered in Klingon society to the point of near-deification, and many aspects of Klingon culture came to revolve around an emulation of Kahless' life. ()

The warrior ethos had been an important aspect of Klingon society since the time of Kahless, but the warrior aspects became much more dominant beginning in the early 22nd century. Over time, the warrior caste gained greater prominence to the point where the Klingons widely came to be regarded as a "warrior race." () 

The Klingons' relationship with Humans and the Federation was rocky at best. Following the disastrous first contact between Klingons and Humans in the Broken Bow Incident, tense rivalries and unavoidable conflicts often developed between the two races. (; ) In the year 2154, the Klingons gained access to the genetic material of Human Augments and tried to adapt this genetic engineering to improve themselves. The test subjects did gain increased strength and intelligence, but then, their neural pathways started to degrade and they died in agony. One of the subjects suffered from the Levodian flu, which was modified by the Augment DNA to become a fatal, airborne, mutagenic plague that spread rampantly through the Empire, from world to world. In the first stage of this plague, Klingons lost the ridges on their foreheads and began to look more Human. With the help of a Klingon scientist named Antaak, Dr. Phlox of the Earth starship  was able, using the DNA of Captain Archer, to formulate a cure that halted the genetic effects of the virus in the first stage. This retained the changes in appearance, along with some minor neural re-ordering. The neural ordering caused changes in the emotional make-up of the Klingons. For example, the infected started to feel fear. Even though the infected did not develop any stage-two characteristics – such as enhanced strength, speed, or endurance – they did not die from it. This left millions of Klingons changed. These alterations were even passed on to their children. () 

In the 2250s and 2260s, groups of Klingons with and without ridges had been encountered by the Federation. (; ) By the 2270s it had become more commonplace to encounter Klingons with forehead ridges than not. (, et al.) Klingons did not discuss the circumstances of this mutation with outsiders and, by the 24th century, the reason for smooth-forehead Klingons was not widely known outside the Empire. (; )

By 2223, relations between the Federation and the Klingon Empire degenerated to a point of relentless hostility, which lasted for several decades. (; )

The lingering tensions between Klingons and Humans continued to rise, eventually leading to the Battle of Donatu V, near Sherman's Planet in 2245 and the Battle of the Binary Stars in 2256, which was provoked by T'Kuvma in order to unite the squabbling Great Houses against the Federation. (; ) This resulted in the first , where the Klingon Empire successfully invaded and occupied nearly twenty percent of Federation space. ()

Later, another conflict erupted into the  of 2267. The Organians quickly intervened and ended the war after only four days of fighting. () Over the next several decades an uneasy peace developed, broken only by brief but fierce skirmishes and conflicts. (; ) A true and lasting peace finally came in 2293, with the signing of the Khitomer Accords, thanks to the efforts of Chancellor Gorkon and the Human Starfleet officer James T. Kirk. (; ; ; ) Since then, despite several periods of rocky relations (see Federation-Klingon War (2372-73)), the Federation and the Klingon Empire have been steadfast allies, especially in the face of Dominion aggression in the 2370s. ()

The Klingon relationship with the Romulan people was extremely erratic. The Klingons typically regarded the Romulan Star Empire as a "blood enemy" since at least the 23rd century. Sporadic Romulan attacks against Klingon colonies, such as the Khitomer Massacre, and interference in Klingon affairs, including the Klingon Civil War, continued to sour relations between the two peoples. (; )

Parallel universes
In the mirror universe, the Klingons were one of the races subjugated by the Terran Empire. () By the 24th century Klingons and Cardassians founded the Klingon-Cardassian Alliance and eventually defeated their  occupiers. ()

In the alternate reality, in 2233, the crew of the  briefly considered whether a particular lightning storm effect, observed by the Kelvin near the edge of Klingon space, might be Klingon in origin. Starfleet discarded this possibility. The phenomenon preceded the arrival of the Romulan mining vessel Narada from the prime universe. The Narada attacked and obliterated an armada of 47 Klingon warbirds in 2258. ()

By 2259 in the alternate reality, after Starfleet's first contact with the Empire, the Klingons had conquered and occupied two planets known to the Federation and fired on Starfleet ships half a dozen times. Tensions between the two powers were high and an all-out war was considered inevitable. During that year, before surrendering to the Federation,  destroyed three  patrol ships on the Klingon homeworld, killing the crews of the vessels. ()

According to , the Klingons joined the Federation by the 26th century in one possible timeline. ()

Society

Klingon society was extremely complex. Before its decline in the mid-22nd century and again in the late 23rd century, Klingon society was based on a feudal system organized around traditional Great Houses of noble lineage, to which various parts of the population owed fealty. The Great Houses are traditionally represented in the Klingon High Council, which was led by a Chancellor, replacing the heredity leader of Emperor.

The decline of Klingon culture was demonstrated in the acts of the Klingons themselves. They stopped caring about their weapons to the point that they let them rust and even stopped caring for true honor. () Sometime after the augment virus took hold of the Klingon Empire, a new regime took control, turning the Empire into an authoritarian state that kept tabs on all who served. (; ) The old ways returned in the latter 23rd and early 24th centuries respectively.

Males traditionally dominated public life in the Empire, assuming the leading roles in politics and the military with only rare exceptions. () There were three notable exceptions to the prohibition of women serving on the High Council. In 2257 L'Rell rose to High Chancellor and Dennas served on the High Council. The third instance occurred in 2293 when Azetbur became Chancellor of the High Council after her father, Gorkon, was assassinated (). Women traditionally dominated the household and the management of the family's affairs. () Klingon women were treated as equals, except in politics and matters of inheritance. Law prohibited them from serving in the High Council and they could not take control of their Houses unless they had the money and no male successors of the lineage. Otherwise, Klingon women were expected to exhibit the same physical prowess and lust for blood and honor as the men.

Klingon society functioned through a system of family reputation and honor. Tradition was an integral part of their lives and breaking from observances was considered a grievous insult to society, an insult not forgotten easily. An offense usually brought shame to the offender's name for several generations. The highest shame was discommendation, an action by the High Council to officially strip a Klingon of his personal or family honor. Bloodlines and relations were also taken very seriously by any "true" Klingon. Lines comprised more than mere family members. ()

The military was integral to Klingon society, as it provides opportunities for warriors to die in battle. For the same purpose, the Empire often seeks to expand through conquest. Discipline was strictly enforced, by one's superiors and subordinates both; the latter may assassinate his superior and take his place. This gives Humans the false impression that there are no rules, but in fact, this can be done only under certain conditions, dereliction of duty and cowardice among them, and the challenge can be made only to a direct superior. ()

An integral part of tradition was the various rituals that marked milestones in a Klingon's life or the history of the Empire. Most notable of the rites was the Rite of Succession, which a future leader of the Empire had to complete with a valid Arbiter of Succession (Captain Jean-Luc Picard, in the case of Gowron) overseeing the proceedings. Before the Rite could begin, there was another elaborate ceremony needed to confirm the death of the previous leader. This was known as the Sonchi ceremony. () Individual Klingon warriors were expected to go through the Rite of Ascension to be recognized as a full adult. () If the house that an individual Klingon belonged to was dissolved or fell into dishonor, he could be adopted into another house through the R'uustai or alternative ceremonies that symbolically marked the joining of kinship and allegiance. (; ).  The Right of Vengeance, allowed Klingons to seek redress for the deaths of their family members by engaging the one responsible in single combat.  The right was so important, that even the discommended could claim it against important political figures, as the discommended Worf was able to challenge Duras (a candidate for the title of Chancellor. ()

Klingons were extremely territorial. According to the first known Klingon linguist, there was no such thing as an "insignificant corner of Klingon space". ()

Evolution

Origins

Like their Human, Cardassian, Vulcan, and Romulan counterparts, Klingons and many of the Milky Way's species all shared a common ancestry back to the ancient humanoids and the DNA code they produced and seeded across the galaxy. Though each evolved under separate conditions from each other, their shared DNA allowed these species to remain roughly compatible and viable with each other. (; ) 

Possessing extreme strength and speed, early Klingons were quite large, weighing up to 200 kilograms. They were armored with an exoskeleton, with spines and bony plates protruding from their sides, upper arms, and back, arms ended with clawed pincers. On their face were venom sacs, filled with bio-acidic compound and spitting glands inside their mouth that were used for defense and to mark territory. Along with these sacs, these early Klingons had two mandibles protruded from their lower jaw. ()

These prehistoric males used vocalizations to frighten other predators, mark its territory, and commence its mating process. These early Klingons mating rituals involving biting its intended mate, to induce pheromone production in the sebaceous glands. Once the mate was ready, the pheromones drew the male Klingon back to its location. ()

The Modern Warrior
Modern Klingons developed a varied line of physical attributes, ranging from well pronounced cranial ridges to what was best described with the dysphemism "pointy-headed", as coined by Harry Mudd.

Anatomy

 
Like most humanoids, the essence of the Klingon anatomical form consisted of a head, neck, torso, and four limbs. 

Internally, the Klingon anatomy was markedly different from that of other humanoids. There was a great deal more multiple redundancies in their organs, a principle they called brak'lul. This allowed Klingons to survive severe injuries in battle. They had twenty-three ribs, two livers, an eight-chambered heart, three lungs, two urinary tracts, and even redundant neural function as well as multiple stomachs. Some geneticists believed that the extra organs, notably the third lung, evolved to give Klingons greater stamina on the battlefield. Klingons had relatively little knowledge of their own biology and their medicine was very poorly developed. This was largely due to their warrior traditions – a Klingon who was wounded was expected to be left to either survive through his own strength, die, or undergo the hegh'bat, a form of ritual suicide. (; ; ) Conversely, their society having dueled and killed each other for generations taught them what is immediately lethal for their species. (; ) 

The most distinctive feature of the modern Klingon was a sagittal crest, beginning on the forehead and often continuing over the skull. (, et al.) Klingons head shape differed between individuals, with some having skulls that extended backwards into an elongated cone shape. () The cranium, itself, was encased in an exoskeleton, which possessed a feature known as the tricipital lobe. () 

The lower half of the face tended to follow a familiar humanoid appearance, and even the ridges on the back of their heads if any tend to be less pronounced and slight, leaving the general shape as Human skull and covered in more skin than bone (see  for example), it was in these regions where hair was most likely seen to grow. Eyebrows tend to grow in a more diagonal direction than other humanoids, but also had noticeably diagonal suborbital ridges where eyebrows otherwise existed.

 

Klingons ears included both external auricle and pinna and recessed pinna. External auricle structures tended to be rounded, and the recessed pinna tended to come to a point before forming into the back of the head, Klingons of the alternate reality had pointed auricles of a similar size and shape to the recessed pinna, with slight ridges along the top. ()

Klingon eyes were generally round in size and shape (consisting of varying shades with white sclera). They may flash more white of their eyes which they use to intimidate a rival or enemy. (; ) Other Klingon eyes had large corneas, pink sclera, and almost gem-like reflective quality to them. (, et al.) Klingons, especially females, were said to lack tear ducts. () However, for males Klingon myth stated that Kahless once filled the ocean with his tears, and at least one Klingon, Kurn, produced tears. (; ) 

They also had more physically pronounced cheeks, and a vertical ridge that went down their chin into their neck which expanded into ridges on their upper chest and shoulders and between the breasts of at least the females. The neck structure varied from smooth flesh to flesh containing pits and bony structures that led up the side of their head, and down the back of their head and spine. 

Klingon noses tended to vary having between several slight horizontal ridges on the brow of their noses, or a single vertical ridge or crack from the top of the brow to the tip of the nose to being wider and flatter, nearly flush against the forehead. They also had heavier ridges over their nostrils, compared to the two minimally ridged or un-ridged nostrils. The nose tended to lay flatter on the face than in other Klingons as well and wider. (; )

Klingon teeth typically consisted of two sharp fangs (incisors) with crooked or sharpened teeth (it was said they sharpen their teeth, often before going into battle). (; ; ) However, not exclusive, others had pronounced thick lips with sharp teeth, with each tooth being considerably larger size. (, et al.)

Klingons of both sexes usually had hair on their heads while male Klingons often had facial hair as well. (; ) Hair was usually long, curly and wavy, described as thick and luxuriant. () Accelerated hair growth was often experienced by Klingons during puberty, or jak'tahla, along with severe mood swings and unusual aggressive tendencies. () Klingon hair greyed with age. () Other Klingons were completely bald, lacking facial hair. (, et al.) For Klingons with hair, cases of receding hairlines or full or partial baldness existed during 2160s, and later in the 2260s through 2290s (; ; ; ; ; ) Klingons as young as those who had reached the Age of Ascension, typically did not wear beards. (; ) Klingons of the alternate reality vary from balding and no facial hair, to those with long hair and beards. ()

Some Klingons also had sharp thick talons on their hands that make up the end of their digits. 

The chest of the Klingon males appeared to be largely smooth and slick, while the females had a large chest area with ridges that came down to the top of their breasts across their shoulders. The backs of these Klingons were highly covered with ridges from shoulder to shoulder and across the small of their back radiating from highly defined and thickly ridged spines. The sides of the abdomen of at least the females appeared to be highly defined with rib-like ridges as well. The buttocks region was even heavily ridged. ()

Klingons had ridged spines, chests and feet (though these ridges and other armor like structures extend out covering almost the entire back, sides and abdominal region on the 'pointy-headed' Klingons). (; ; ) After birth, some Klingon infants experienced a pronounced curvature to the spine, a form of scoliosis, which was correctable by surgery. This "defect" tended to run in Klingon families, especially among females. Federation medicine, fortunately, advanced beyond that, allowing an additional choice of treatment involving genetic modification of the fetus. ()

Augment virus

Klingons afflicted with the augment virus, propagated by a mutation of the Levodian flu was inadvertently created by Klingon researchers who were attempting to bio-engineer enhanced warriors using DNA from genetically-modified Human embryos left over from Earth's Eugenics Wars. These Klingons lost their cranial ridge features, and were in essence, remarkably similar in appearance to modern Humans. (, et al.)

Though the cure for the virus had been distributed throughout the Empire, the afflicted Klingons' DNA had been altered by the virus, and condition was passed onto the descendants of the infected. Initially there were millions of Klingons who had to live with the disfigurement for well over a century. (; , et al.)

Though Human in appearance and distinguishable primarily by tricorder readings alone, Tribbles were still able to single out and identify these Klingons. ()

Hybridization
Klingon pregnancies normally ran thirty weeks, but with mixed species, gestation times were shorter. As early as seven weeks (at which point a Human fetus doesn't even have legs) a fetus with a single Klingon grandparent was already able to kick the uterine wall hard enough to be felt externally. ()  Klingons were said to have a different, not fully compatible number of reproductive organs than Humans. () However, basic anatomy does actually appear to align between Humans and Klingons. () Whenever Klingons interbred with other species they showed characteristics of both species. 

The odds against Klingon-Human conceptions were rather high. However, when successful, Klingon and Human metabolisms sometimes clashed, causing biochemical fluctuations in the mother, which may lead to fainting. Klingon traits remained dominant for several generations, even with a single ancestor; therefore, a child even ¼ Klingon still possessed forehead ridges. It is possible to see the eventual phenotype of a hybrid while the fetus is still gestating using a holographic program and projection. If early enough in development, genetic manipulation can be taken to remove Klingon DNA from the developing fetus. () 

Hybridization revealed that Klingon DNA tended to be more dominantly expressive in the physical appearance of their offspring. 

This was found to be especially true in the case of hair color and appearance of cranial ridges, especially with Humans. (; , et al.) In the case of Romulan-Klingon hybridization, the ears were the only visible telltale indication of their non-Klingon heritage. ()

Physiology

Skin colors ranged from olive, brown, and black to pearlescent/metallic across the entire spectrum of the rainbow, varying from purple, pink, burgundy red, yellowish tan, dark orange, umber brown, grey, blue, dark green, coal black, and chalky white. Their skin had a shimmery slick metallic semi-reflective sheen to it. Albino Klingons were often considered outcasts in Klingon society. ()

Klingon children matured far more quickly than Human children. At the age of only one Earth year, a Klingon child had the appearance a Human child had at about four. By the age of eight Earth years, a Klingon attained the maturity a Human did not reach until about age sixteen. (; ) When Klingon children began growing into adults, they went through jak'tahla, a Klingon form of puberty. () Like other mammalian species, Klingon females were capable of lactating to breast-feed infants. ()

Klingons tended to live for over 150 years. Even into advanced old age, Klingons were usually still strong enough for combat, but were still susceptible to various mental conditions associated with old age. () 

Klingons were notably stronger than most humanoids, at least as strong as Vulcans. However, despite their endurance and general robustness, they had a considerably lower tolerance to the cold than humans. (; )

The Klingon sense of smell was notably acute; in fact, it was powerful enough to be used effectively to track down prey during a hunt. Their sense of smell was also suggested to play an integral role in their mating practices. ()

Doctor Julian Bashir once sarcastically noted that the natural odor produced by Klingons was comparable to an "earthy, peaty aroma with a touch of lilac." () To Humans and Vulcans alike, Klingon ships smelled bad. (; ) For example, Leonard McCoy once referred to the environment of a Klingon Bird-of-Prey, operated until recently by Klingons, as having a "stench." ()

Klingons such as Kurn had the instinctive ability to sense the decision to kill by looking into the eyes of their opponents. () Worf despite being raised by Humans on Earth, retained this ability when he sensed Martok's warrior spirit returning enough to win their duel without killing Worf. ()

Klingon blood could contain ribosomes that were compatible for transfusion with a Romulan. () Klingon blood varied in color from dark red to more of a lightish pink. (, )

Klingon nutritional requirements were typical of most humanoids, whereas, Dr. Pulaski once noted that, while their food was considered by most to be unpalatable, usually, "what kills us, kills them." () One exception of this observation was the tea used in the Klingon tea ceremony, which was toxic to the point of being deadly to Humans, and capable of seriously sickening Klingons, as it did. ()

Religion and tradition

See also: Klingon religion

Ritual was a very important element in Klingon society.

Klingons did not believe in fate; however, they did appear to believe in some form of luck. ()

A Klingon who was unable to fight, and hence unable to live as a warrior anymore, had the traditional obligation of committing the hegh'bat, which was the Klingon ritual suicide. Tradition dictated that the eldest son or a close personal friend must assist. That person's role was to hand the dying Klingon a knife so that he could plunge it into his heart, remove it, and then wipe the blood on his own sleeve. ()

Klingon rituals included the R'uustai, a bonding ceremony which joined two people together in a relationship similar to brotherhood. () Klingon tradition held that "the son of a Klingon is a man the day he can first hold a blade." ()

If a Klingon warrior struck another Klingon with the back of his hand, it was interpreted as a challenge to the death. Klingon warriors spoke proudly to each other; they did not whisper or keep their distance. Standing far away or whispering were considered insults in Klingon society. ()

According to Worf, taking hostages was considered by Klingons to be a cowardly act. Although, there was an incident in which a Klingon aimed his disruptor at the Enterprise-D's warp core, thus taking the ship hostage, in effect. ()

When choosing a mate, it was traditional for a female Klingon to bite the male's face, allowing her to taste his blood and get his scent. Males were also known to bite the face of the female they were interested in. ()

A Klingon can divorce their mate by backhanding them across the face and saying “N’Gos tlhogh cha” which means “our marriage is done”.  (Star Trek Adventures: “the Klingon Empire Core Rule Book)

Worf once told Wesley Crusher that, per Klingon mating rituals, "Men do not roar. Women roar. Then they hurl heavy objects. And claw at you." Of men, Worf said, "He reads love poetry. He ducks a lot." () Klingon daughters traditionally were given a piece of jewelry called a jinaq when they became old enough to select a mate. ()

Long hair was important part of the ancient traditions of Klingons as told in the legend of Kahless; it was said he took a lock of his long hair thrusting into the caldera of the volcano of Kri'stak, where it began to burn, and then after plunging it into the lake of Lusor, he molded and twisted into his legendary Sword of Kahless, which he used to kill the tyrant Molor, and then gave it the name Bat'leth, the sword of honor. () To grow a beard was an ancient tradition, to Klingons in the 24th century especially it was a symbol of courage, which they were against shaving (though they were okay with a trim). Klingons did not like having too much hair removed either, letting it continue to grow, and only a little bit of trimming. ().

Science and education
As of the 2150s, the warrior caste had a dominant role in Klingon society, causing science and education to be neglected virtues. Nevertheless, ' father was a teacher and his mother a biologist at a university. Antaak, on the other hand, was disowned by his father when he chose to become a "healer", i.e., a doctor. In 2149, he was an expert in metagenic research and attended an Interspecies Medical Exchange conference, albeit in disguise, as the Klingons were not invited. By 2151, Klingons possessed technology to encrypt intelligence data within an operative's DNA. ()

As of 2154, medical research was not considered "a priority" for the Klingon High Council, which was why the Empire did not possess the medical expertise to confront the Augment virus without assistance. ()

By the late 23rd century, the Klingon Empire sent out expeditions, one of which made first contact with the Ventaxians in 2297. Hired Klingon craftsmen went on to construct homes on Ventax II. ()

As of 2365 and onward, Klingon vessels had dedicated science stations. (; )

In 2369, Kurak was a warp field specialist from Qo'noS. While she was a brilliant expert in the field of subspace morphology, her efforts went unappreciated by her fellow Klingons. ()

Although the concept of a healer tending a warrior's wounds after a glorious battle was considered sufficiently honorable to be mentioned in songs (e.g., Dr. Bashir and Worf at Internment Camp 371), Klingon medical expertise kept having a bad reputation in the 24th century. In 2367, Lt. Cmdr. Hobson of the  remarked that no one would ever suggest a Klingon to be a good ship's counselor, as he considered them unsuited for such a position. In 2374, Klingon General Martok preferred to be treated by the Human Dr. Bashir, opining that "Klingons make great warriors, but terrible doctors." In 2376, when Lewis Zimmerman was faced being treated by an "obsolete" EMH Mark I, he claimed he would be "better off being treated by a Klingon field medic". In one particularly extreme case, the crew of Kohlar's battle cruiser, a Klingon generational ship, lived with the Nehret, a fatal virus, for over a century, without ever even considering it might be an actual illness, instead believing it was simply "old age" that kills those unlucky enough to not die in combat. (; ; )

In the year 2404 of an alternate timeline that diverged in 2378, Klingon scientist Korath created the chrono deflector, a time travel device. ()

Technology
According to Quark, Klingons achieved warp drive sometime after 1947. By the year 2152, Klingon vessels were capable of warp 6. (; ) 

As of the early 2150s, Klingon vessels were comparatively advanced. In contrast to their Earth Starfleet counterparts, Klingon ships were equipped with photon torpedos, deflector shields, tractor beam emitters, and had thicker hulls reinforced with a coherent molecular alloy. However, as of 2151, Klingons had no knowledge of holodeck technology. ()

By 2256, cloaking screens began to be used on Klingon vessels, such as the Sarcophagus, but Starfleet was quickly able to penetrate them. As of 2268, the Klingon D7 class battlecruiser design was used by the Romulans. In 2269, Starfleet assumed at least Cmdr. Kor's  to possess effective cloaking capability against their sensors, a technology thought to be uniquely Romulan the previous year. Kor would later reminisce that, when his ship was equipped with a cloak, it was still a new piece of technology for the Klingons, understood only by a handful of engineers in the Imperial Fleet. By 2285, at least one more Klingon ship was definitely using a cloaking device. (; ; ; ; )

As of 2367, Klingon transporter systems had a range of 20,000 kellicams, which was a common Klingon unit of length measurement as early as 2285. (e.g., ; )

See also

Klingon starship classes
Klingon starships
Klingon blade weapons
Klingon language

People

Named

List of Klingons

Unnamed

List of unnamed Klingons

Klingon space

Food and beverages

Appendices

Appearances
A list of all appearances of Klingons (excluding appearances of Worf and B'Elanna Torres).

 (Season One)
 (Season Two)

  (Season Three)

 (Excalbian recreation only)

 (Borg drones)

 (illusion only) (Season One)

 (Season Two)
 (holograms only)

 (archive footage only)
 (hologram only) (Season Three)

 (Season Four)
 (hologram only)

 (Season Five)

 (Season Six)

 (Season Seven)

 (Season One)

 (Season Two)

 (Season Three)

 (Season Four)

 (Season Five)

 (Season Six)

 (Season Seven)

 (Season Three)
 (hologram only)
 (hologram only) (Season Four)
 (hologram only)
  (hologram only)
 (Klingon Borg)
 (photo only)
 (dream only) (Season Six)
 (Klingon Borg)
 (Klingon Borg) (Season Seven)
 (hologram only)

 (Season One)

 (Season Two)

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 (Season Two)

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Background information

The Klingons are the only non-Federation species to have appeared in twelve films. In , they attempt to attack V'ger and are vaporized. They appear in the simulation of the Kobayashi Maru test in . They are the primary villains after the Genesis technology in . In , the primary vessel is a Klingon Bird-of-Prey and the Klingon ambassador appears early on, to have Kirk extradited. In , a Klingon ambassador is featured again, on Nimbus III, and they also appear as secondary villains, as a young brash Klingon officer chases Kirk for the glory of defeating an infamous enemy of the Empire. In , General  is the primary villain as a Klingon and they also appear throughout the film during the peace process of the Khitomer Accords.

In all TNG movies, Worf appears despite his commitments to Deep Space 9, and the Duras sisters appear in . At least three Borg drones seen in  are also of Klingon origin.

 is currently the only series in which no Klingons appear (though an image of Klingons from a TNG episode does appear in first episode during a news report). Martok’s skull appears in season 2.

The Klingon ships once again appear in the Kobayashi Maru scenario in the film . Their full appearance was cut from the film, their prevalence up to then leading Damon Lindelof – who worked on the movie as a producer and a writer of the film's screenplay – to remark, "What Trek film would be complete without Klingons? The answer is... ours." (deleted scene "Klingons Take Over Narada" audio commentary,  and Three disc Blu-ray)) Klingons were shown on Ketha Province of Qo'noS in . Klingons do not appear in  however in Star Trek Beyond - The Makeup Artistry of Joel Harlow, it is revealed a Klingon was intended to appear in the film, but ultimately cut.

Alex Kurtzman has explained that if Worf is ever introduced into Star Trek: Picard, he would look the same as he did in previous shows, and explained that Discovery Klingons differences are regional racial differences like the differences between Romulans is explained in Picard.  
"No, he's got to look the same," Kurtzman says. "We're not changing that. It's funny, because you'll see by the time you get to [Picards] third episode, there are different kinds of Romulans that look different, depending on their territories. You'll see some Romulans that are just about the ears, you'll see some with the ears and deeper ridges in their foreheads... so I have to believe that there are different iterations of Klingons, depending on where they are from."

Apocrypha 
In the novel Summon the Thunder, part of the Star Trek: Vanguard series, the Klingons who had a Human appearance (descendants of the victims of the Klingon Augment virus) are referred to as "QuchHa", or "the unhappy ones". They usually served in their own units although they also were known to mix with the rest of the fleet on occasion.

"Against Their Nature", the first installment of "Star Trek: Klingons - Blood Will Tell", an IDW Comics series which tells the stories of , , , and  from the Klingon point of view, suggests that, while Phlox and Antaak's cure removed Augment strength and Augment intelligence, those affected retained the superior ambition of Augments, and as such these Klingons were largely responsible for the Empire's expansion in the century between  and TOS, eventually becoming powerful enough to achieve a majority on the High Council.

In the novel Pawns and Symbols, Klingons are discovered to be color blind in the Human sense, unable to distinguish red from black. It is also discovered that their vision extends into the ultraviolet, to 32,000 Ångströms.

In the novel , the Klingons are described as having been economically conquered and uplifted by the Karsid empire. The Klingons then rebelled and overthrew the Karsids, obtaining their high technology. This was given as one reason for why the Klingons were the way they were, and also how they could have developed star-faring technology given their current social structure.

In Star Trek Online, most Klingons are once again enemies with the Federation by 2399, having taken advantage of the Romulans by conquering much of their territory in the wake of the death of Shinzon and then the destruction of Romulus. The Klingons have also conquered the Gorn, the Orions, and the Nausicaans. Evidently, Klingons are seen joining Starfleet, if unlocked. The Federation-based mission "Past Imperfect" gives a possible answer to how the Augment virus' alterations was eventually cured by revealing that the mad Klingon Admiral B'Vat had kidnapped Miral Paris and brought her to the past, where they used her Klingon/Human DNA to manufacture a cure.

In the online game Star Trek Timelines, includes all three types of Klingons appearing from their various respective eras accurately portraying each character's respective and different physiological differences in appearances. Kor, Koloth, Kang appear reflecting the 23rd century TOS era "Human-augment" part of the history, T'Kuvma appears representing the 23rd century era Klingon/Federation war, characters such as Chang, Kruge, Colonel Worf representing the 2270s-90s period, characters such as Gowron, Worf, Martok, Alexander, B'Elanna Torres, and Duras Sisters appear reflecting the 24th century. 

The Discovery related event which introduces T'Kuvma and involves Katherine Janeway (including a 24th century Klingon variant Janeway) is described as such "The USS Shenzhou has arrived in the 24th Century, engulfed in a destructive temporal anomaly. Starfleet has yet to make contact with the crew, but the anomaly has brought all manner of things from the Shenzhou's time into the present day..."

External links

 – dedicated Klingon search portal

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