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A holodeck on the Enterprise-D; the arch and exit are prominent.

The holodeck is a simulated reality facility located on board starships and starbases in the fictional Star Trek universe. The holodeck was first seen in the pilot episode of Star Trek: The Next Generation, "Encounter at Farpoint". An episode of Star Trek: The Animated Series, "The Practical Joker", had anticipated the idea by portraying a recreation room capable of holographic simulations. The concept of a holodeck was first shown to Humans through an encounter with the Xyrillan race in the Star Trek: Enterprise episode, "Unexpected".

According to the Star Trek: The Next Generation Technical Manual, the holodeck is an enclosed room in which objects and people are simulated by a combination of replicated matter, tractor beams, and shaped force fields onto which holographic images are projected. Sounds and smells are simulated by speakers and fragranced fluid atomizers, respectively. The feel of a large environment is simulated by suspending the participants on force fields which move with their feet, keeping them in one place (a virtual treadmill). Perspective is retained through use of sound damping fields and graviton lenses, which make objects, people, and sounds appear to be more distant. The effect is an ultra-realistic simulation of an environment, with which the user can interact.

Most holodeck programs shown in the episodes run in first person "subjective mode", in which the user actively interacts with the program and its characters. The user may also employ third-person "objective mode", in which he or she is "apart" from the actual running of the program and does not interact with it (all of the program's characters will ignore the user as if they were not there - this was shown in the Enterprise episode "These Are the Voyages..."). The user may then take more time to observe the trappings of the program, and even have conversations with other users, without interrupting the flow of the program itself.

Matter created on the holodeck requires the holoemitters to remain stable and will quickly disintegrate if it is removed from the holodeck, unless working with a mobile emitter, although nitpicker Phil Farrand has noted in Nitpicker's Guides various episodes where the writers forget this.

According to the Starfleet Survival Manual, holomatter can be disrupted using an inverse photonic pulse from a phaser, which destroys the holomatter while leaving real matter unharmed. This method could be used to escape a holodeck by repeatedly firing until the real door is revealed as demonstrated in Star Trek: Insurrection.

User controls for a holodeck are typically located near its door (inside and out); an arch surrounds the exit and can be summoned by a user to start, modify, or stop a simulation (as shown on the Enterprise-D in TNG). The holodeck includes safety protocols to protect the users. While it does not entirely shield users from minor injury (such as strained muscles or dislocated joints), it does prevent more serious injuries and fatalities. The protocols were designed so that users could derive maximum use from the holodeck with a high degree of realism and perceived jeopardy. The safety mechanism can, however, be disabled intentionally or as a result of malfunction. This has the potential to allow a user of the holodeck to be seriously injured. In several episodes, it is shown that disabling the safety protocols on a holodeck normally requires the authorization of at least two senior Starfleet officers.

The Emergency Medical Hologram is a spin-off from holodeck technology.

Two of the main purposes of the holodeck as shown in the various episodes are recreation and training. In several episodes they are also used to recreate a crime or questionable incident to determine the forensics and logistics of it for law enforcement purposes. However, the technology is also used for morally questionable ends, such as the holosuites that are owned and rented out – often for sexual purposes – by Quark on Deep Space Nine.

Holodecks appear to be vulnerable when damage occurs to the computer controlling the holodeck. Malfunctions have at times resulted in the safety protocols being inadvertently disabled. Some malfunctions have also been known to trap users inside the holodeck. This happened in the first episode featuring a holodeck (The Practical Joker (TAS episode), non-canon, where it was called a recreation room.) This also resulted in the shooting of the ship's historian on board Enterprise in one instance, and several crew members were trapped. In the episode "A Fistful of Datas", Lt. Worf, his son Alexander Rozhenko, and Counselor Troi were trapped in an 19th century American West adventure with the safety protocols disabled when a computer experiment involving Lt. Cmdr. Data went awry. Worf received a minor gunshot wound when the computer began remaking all the characters as replicas of Data. However he was able to safely play out the story, and once the story ended the trio was able to leave the holodeck. On at least one occasion the disabling of a Holodeck's safety protocols has been used as a tactical advantage in a hostile encounter. In the Holodeck Club scene in Star Trek: First Contact, when under attack from the Borg, Jean-Luc Picard disables the security protocols and grabs a holographic Tommy gun from a nearby violin case, shooting and killing several Borg (Picard explains that with no safeties in place, holographic bullets become as deadly as the real thing).

Writing stories and plotlines for the holodeck is an activity pursued by people known as holonovelists. It was the chosen profession of Lt. Tom Paris of the USS Voyager, and he pursued it when the ship finally returned from the Delta Quadrant in the show's finale. In real life, some fans have criticized the series from TNG forward for having too many episodes with holodeck malfunctions.

• In the X-Men comic series, the X-Men train in the Danger Room. In the film X-Men: The Last Stand, the Danger Room is shown as holographic, very similar to Voyager's holodeck.
• In the science fiction television series Stargate SG-1 and Stargate Atlantis, some advanced races (such as the Asgard and the Ancients) have holo technology.

• Artificial reality
• Mixed reality
• Augmented reality
• CAVE - Cave Automatic Virtual Environment
• Methods of virtual reality
• Narrative environment
• Simulated Reality
• Virtual retinal display
• Volumetric display
• Omnidirectional treadmill
• Haptic

• P. Farrand, Nitpicker's Guide for Deep space Nine Trekkers New York: Dell (1996): 44 - 47
• R. Sternbach & M. Okuda, Star Trek: The Next Generation Technical Manual New York: Bantam Books (1991)

• Holodeck article at Memory Alpha, a Star Trek wiki
• Holodeck at STARTREK.COM
• Holodeck and Computers FAQ by Joshua Bell