In the 24th century, almost any experience or enviroment can be recreated, thanks to the development of holotechnology.

Psychologists realized long ago that being confined in a starship for extended periods of time can be mentally unhealthly. Starfleet worked for years to create an energy-efficient, virtual-reality system to alleviate the tedium of interstellar travel, and eventually came up with a Holographic Enviroment Simulator. This form of holotechnology is commonly known as a holodeck, and can be used as a means of education, entertainment, or simply relaxation. Holodeck technology became practical in the mid 24th century, at about the time Galaxy-class ships were created, and quickly became a requirement for deep space Starfleet starships.

When the holodeck is not in use, it is simply a large, empty room with gridlines dividing the interior. However, when activated, a holodeck can, through a combination of transporter-based matter replication and three-dimensional image projection, simulate almost any enviroment or person.

A holodeck uses two main subsystems to create artificial enviroments, an imagery subsystem, and a matter conversion subsystem. The holodeck imagery subsystem creates background scenery in a simulated enviroment. This works in two ways: first, it generates 3D projections of distant objects - city lights, rolling hills, a blazing sun - in a sophisticated extension of forced perspective. Secondly, this subsystem uses forcebeams to give nearby objects the appearance of substance. The matter conversion subsystem uses both transporter and replicator technology to create real matter within the holodeck. This same technology is also used to create lifelike interactive beings. These characters have real physical substance and are controlled by discreet forcefield and tractor beams. Used in conjunction, these two subsystems can create a fully interactive enviroment, so likelike as to be indistinguishable from the real thing. For example, the imagery subsystem can generate an image of a telephone on a desk; if a user is about to pick up the receiver and dial, the matter conversion subsystem creates an actual telephone. The main components of the holodeck are micro-miniature omni-directional holo diodes (OHDs). Each size-sided OHD contains an optic section and a forcefield section. Orchestrated by a computer, the OHDs create a world that appeals to all five senses, as simulations can project sounds, smells, and tastes using speakers, atomizers, or replicated matter. The more complex the similation, the more memory and power are required from the ship's computers.

Holodecks are used by most Starfleet crew members, who tend to program their favorite simulations into the computer so they can return to them over and over. Classic novels, periods in history, or famous locations are just some of the simulations available on a holodeck. For instance, Captain Picard of the USS Enterprise NCC-1701-E used the holodecks to relive the adventures of his favorite fictional detective, Dixon Hill. Data also liked to use the holodeck for sleuthing; his personal favorite was perhaps the most famous detective in literature - Sherlock Holmes.

There are normally three sets of controls for a holodeck. A small command console allows users to control the holodeck before entering. Traditional voice commands can stop, freeze, replay, or adjust any situation. The final control option is via the holodeck arch controls. The arch controls can appear within an ongoing simulation and provide communications and non-holodeck computer functions. Chief among the holodeck etiquette rules is a ban on using a living person - especially someone serving on the same ship - as a model for personal fantasy fulfillment. However, there is nothing within the holodeck systems that prevents anyone from doing just that.

Although a crew member can still break a leg while skiing, for example, safety features are built into the holodeck that prevent serious injury or death. Safety systems can go offline, but this is unusual, and is normally the result of shipwide difficulties. Some individuals establish a psychological dangerous dependency on the holodeck; this condition is known as holodiction. Although not common, it is often very difficult to resolve, especially as holodiction usually masks deeper problems. Shy Lieutenant Reginald Barclay tried to boost his self-esteem by creating programs in which he could intimidate other crew members and which exaggerated his own sex appeal. Happily, Barclay was able to regain his real-life self-confidence and cure his holodiction. Most crew members, however, are able to simply enjoy the ride and use the holodeck for its intended purposes.