Telepathy
Telepathy was the extrasensory form of non-verbal communication that was conducted directly from one mind to another. It was derived from telepathic energy. () An individual capable of telepathy was known as a telepath, psychic, mind reader, or reader of minds. Telepathy existed in various forms across a number of species. A related ability was empathy, the perception of others' emotions.

Overview
The vast majority of telepaths could both send and receive messages. () It was sometimes considered unusual for a telepathic species to experience hallucinations or premonitions. ()

According to Neelix, "telepathy is a message, words, a voice." () Most examples of telepathy involved the exchange of words, paralleling verbal speech. () In other examples, such as the Cairn, telepathy was transmitted via images, allowing for a much faster rate of communication. () The living starship Gomtuu could mentally communicate millennia of experiences in a few seconds. ()

The Enarans could implant their memories into the minds of non-telepaths, allowing those persons to relive their experiences. () While the Ullians performed what they called telepathic memory retrieval, which allowed them to help other individuals recall and enhance moments from their own past. ()

Some telepaths could only communicate effectively with their own species. For example, the attempts made by the beings trapped in a Tyken's Rift in 2367 to communicate telepathically caused mental harm to the crew of the  and could only be perceived by Betazoids through metaphorical dreams. () 

Species such as Betazoids and Vulcans could speak mentally with other telepaths or empaths and also read the thoughts of non-telepathic individuals. Vulcans, however, were rarely seen using telepathy aside from the mind meld. (; ) Betazoids were even capable of perceiving the impulses of non-sentient animals. ()

A touch telepath required physical contact to communicate with the mind of another being. The Vulcan mind meld was a form of touch telepathy. () This form of telepathy could be produced between two completely forms of life, such as between carbon-based and silicon-based lifeforms. ()

Some species, such as Ferengi and Dopterians, could not be read telepathically, which may be attributed to the structure of their brains. (; ) The Sheliak were so alien that they were incomprehensible to telepaths. ()

Natural telepathy
Telepathy was a natural attribute of some species, present in almost all healthy individuals. In other species such as Humans, telepathy manifested only infrequently, like with Miranda Jones. () The Aenar were an unusual example of an all-telepathic subspecies within the non-telepathic Andorians. ()

Over time, telepathy had become the first and natural way of communication for the Cairn, so much so that it later became the only way as they had lost the physical ability to speak. ()

There were species that possessed extraordinary telepathic powers. The Talosians, Melkot, Botha, and the telepathic pitcher plant could manipulate perception or even trap individuals within fabricated mental realities. (; ). The Halanans and the Botha had the ability to create telepathic illusions that interacted with the real world like physical objects. (; ) The Ocampa could sense impressions from the past, future, and alternate timelines. () 
Tarquin's species were not normally telepathic, in fact the odds of being born telepathic on Tarquin's homeworld were one in fifty million. Those that were, were considered to be a threat to the population, and these rare telepaths were sent into exile. These telepaths could read psychic imprints left in objects by those who previously handled them. ()

Because Deanna Troi was a Human/Betazoid hybrid, she was not fully considered a "true telepath". According to Data, "Her skills are empathic in nature. She is able to sense the emotions of other beings," but as well, could communicate telepathically with her mother, Lwaxana Troi, as well as her imzadi, William T. Riker. (, etc.)

Acquired telepathy
Under rare circumstances, individuals could gain telepathic powers. In 2252, Charles Evans was given telepathy and other powers by the non-corporeal Thasians so that he might survive on their world. () 

In 2265, exposure to the galactic barrier caused Gary Mitchell to develop various abilities, including telepathy. () 

Federation scientists at the Darwin Genetic Research Station on Gagarin IV used genetic engineering to create children with telepathy and telekinesis in the mid-24th century. ()

Telepathy and technology
A few civilizations developed mechanical telepathy. Landru, a gifted engineer and philosopher, constructed a computer able to telepathically control most of the population of the planet Beta III. () 

The members of the Think Tank used a device that enabled mental communication, as their species differed so profoundly that they never would be able to communicate by other means. () 

Borg neural transceivers and interlink nodes allowed multiple individuals to share thoughts. () Chakotay's interaction with the Borg Cooperative, as per The Doctor's "best guess", was that "the residual neuropeptides heightened his telepathic receptivity." () 

The enigmatic Flint built a series of robots that could be controlled telepathically and respond to mental emanations consistent with distress or danger. () The Romulan telepresence unit relied on telepathy to control drone ships over interstellar distances. ()

Devices existed to record an individual's thoughts and memories, for example the Federation psycho-tricorder, the Klingon mind scanner, and the Romulan mind probe. (; ) The Breen used cortical implants to achieve similar results. () The psi-wave devices used by the Prytt could read and transmit thoughts, which had the side effect of enabling "telepathy" between individuals implanted with the devices. When Jean-Luc Picard and Beverly Crusher were telepathically linked, Crusher came to "wonder how true telepaths sift through it all. How can they really get to what someone's thinking if the minds keeps churning all this flotsam to the surface?" ()

The Saltah'na created energy spheres that functioned as telepathic archives. The spheres caused those exposed to them to re-enact the thoughts and actions of the individuals stored within. ()

As of 2368, the Federation had no technology capable of blocking telepathic transmissions. () Still, telepathic suppressors were used by other species, and according to Quark, they were typically bulky. ()

The Coppelius android Sutra had enough telepathic ability to learn the Vulcan mind meld technique. ()

Cultural perspectives
Telepathic civilizations often developed a moral code that governed the use of telepathic powers. Several telepathic cultures considered conversing mentally in the presence of non-telepathic individuals to be impolite. (; ) Invading another being's mind against their will was considered to be an act of assault comparable to rape. Some, such as the Ullians, codified this into law. (; ) The Mari policed violent thoughts as rigorously as violent acts. () Information obtained via telepathy was not considered definite evidence in Federation jurisprudence. ()

Some species used telepathy as a means of attack, for example the Melkot, the Letheans, and the Botha. According to Spock, "True telepaths can be most formidable" (; ; ) The ancient Vulcans developed a psionic resonator, the Stone of Gol, that amplified telepathic energies to kill. ()

Honesty was embraced by Betazoid culture since the idea of hiding information among a race of telepaths would be functionally impossible; however honest feedback was almost to a point considered rude by other cultures. Lwaxana Troi, on numerous occasions, commented on her befuddlement at the Human practice of fibbing to spare others' feelings or for politeness' sake. () In something of a similar vein, privacy was a concept unknown to the Cairn before they started to interact with the Federation. ()

Vulcans and Betazoids established long-term psychic mating bonds; these links could be made with non-telepathic individuals. (; ). The non-telepathic Xyrillians also established a mental connection, with the aid of special granules, during their mating process. () Telepathy was used ritualistically by joined Trill to commune with past hosts. ()

Telepathy has allowed forms of interaction that would otherwise be impossible. The Medusans were a non-corporeal lifeform so different that the sight of them drove Humans insane. Interaction with them was very limited until 2268, when Miranda Jones established a telepathic link with Ambassador Kollos, proving that it could be done. () The mute Federation mediator Riva was telepathically linked to a Chorus that allowed him to communicate verbally. ()

Individuals could be prejudiced against telepaths. () The Devore considered trust to be a concept that was alien to telepathic species and therefore deeply mistrusted telepaths, searching all starships passing through their space for them and imprisoning any found. () Tarquin's species regarded telepathic individuals as threats and exiled them to other planets. ()

Draim, a member of the Orion Syndicate, used telepaths to check the loyalty of his underlings. ()

There were very few telepaths in Starfleet. Because of this, even members capable of telepathy usually utilized speech instead. ()

The concept of reading minds could also be used euphemistically, as a Romulan commander did in 2266 in reference to James T. Kirk anticipating one of his tactical moves. ()

See also 
 Extrasensory perception
 Neurogenic telepathy
 Psychokinesis
 Psychoprojective telepathy
 Telepathic mating bond
 Telepathic species

Appendices

Appearances 
This list contains instances of humans or aliens speaking to each other telepathically, reading someone's thoughts, and reading the images and memories of someone's mind. Although empathy and telekinesis are forms of telepathy, they are not listed here. For appearances of empathic powers and telekinesis please see: Empathy and Psychokinesis, respectively. 
 
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Background information
Star Trek science consultant and writer André Bormanis has revealed that telepathy within the Star Trek universe works via the "psionic field." According to Bormanis, a psionic field is the "medium" through which unspoken thoughts and feelings are communicated through space. Some humanoids can tap into this field through a kind of sense organ located in the brain (e.g. the paracortex). In the same manner that Human eyes can sense portions of the electromagnetic field, telepaths can sense portions of the psionic field. 

Joseph Gatt imagined his character, 0718, to be in telepathic contact with the 's systems at all times through cybernetic implants, more precisely a miniature version of the ship's computer implanted in the back of his head.

External link 
 

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Physiology
Psychology