A Holographic Environment Simulator, or holodeck for short, was a form of holotechnology designed and used by Starfleet. They were installed aboard starships, space stations, and at Starfleet institutions for entertainment, training, and investigative purposes. A typical holodeck consisted of a room equipped with a hologrid that enable holographic projections and creation of holodeck matter through the manipulation of photons contained within force fields to be created inside the room. A holodeck combines elements of transporter technology with that of replicators, by generating actual matter, as well as projecting force fields to give the objects the illusion of substance.

It can be controlled from an exterior control or the interior arch control. This arch can be summoned at any time to change the parameters of a running holoprogram. Matter and energy are interchangeable as such objects created on the holodeck can be either matter or energy.This, however, used an extreme amount of power, and caused repetitive problems to occur in its safe usage. Sometime in the mid 24th century, this was slowly phased out in favor of simpler 'true' holographic technology by focusing on the photons contained within micro force fields. This was not only safer and used less power, but had more varied usage and could be easily controlled with quicker reaction times.

Holodeck walls can generate holographic images that appear to extend for an unlimited distance, seemingly much larger than its own dimensions. In doing so, however, the holodeck is aware only of its users; it does not recognize its own created objects. For example, if a person were to throw a holographic rock at the holodeck's walls, the rock would not be allowed to pass beyond the wall. With this, an individual approaching a wall causes an instant shift away. It can also manipulate light photons, 'lensing' them to make individuals appear further away if two persons were separated in a scenario. The holodeck can change gravity in three dimensions, so occupants don't notice the change as observed during the stop of B'Elanna Torres' holographic orbital skydiving session.

Holodecks are equipped with safety protocols to prevent serious injury during their use, though these can be disengaged by the user when required. Holodeck characters have been known to include a program element called a perceptual filter to hide anachronisms to the program's time period, such as uniforms and communicators, and prevent them from raising the character's ire and curiosity. Among the viewing modes on a holodeck is objective mode, in which the user doesn't interact with the characters, and subjective mode, in which the viewer can interact with the characters as well as alter his or her surroundings.