The UAC is an acronym that appears as part of a logo on various textures in the Doom games and stands for the "Union Aerospace Corporation", a fictional multi-planetary conglomerate (possibly a megacorporation) that operates facilities on Mars and its two moons, Phobos and Deimos, where Doom takes place. The facilities on the two moons were used to conduct various classified projects, including experimental inter-dimensional travel through teleporters (called "gateways"). This project grew unstable, and project "volunteers" began to suffer from a violent form of progressive homicidal insanity culminating in a sudden, explosive death. The story of the games begins after distress messages sent from the moons reach Mars, where the protagonist is stationed.
Not much detail is given about the UAC's activities given the sketchy nature of the plot of the games, but it is made clear that before the invasion took place the UAC had been researching technology used for teleportation as a major contractor to the military, which may explain why their facilities were guarded by former marines now turned private military contractors (such as the player character, a marine).
As is evident during progress in the games, the populations of the UAC facilities are either killed or possessed by the hellish invaders unleashed by the teleportation experiments. In Doom II, which takes place on Earth, there are occasional UAC logos throughout the levels, indicating that the protagonist enters various areas owned by, operated by, or associated to the UAC on Earth.
Both games include crates of various sizes containing unknown materials which are marked with the UAC logo. A clear sign that the UAC develops military technology is the "UAC rockets" label on the boxes of rockets the player may pick up as ammunition for the rocket launcher.
The UAC logo is composed of two flattened spherical shapes with a smaller triangular space between them. The acronym is often written beneath the shapes using the customary rounded font of the games.
 The UAC in Doom 3
With Doom 3, a retelling of the original Doom, the story is given greater depth of detail through the player's interaction with human characters, PDAs, cut scenes, and scripted events. The intro to Doom 3 describes the UAC as the largest corporate entity in existence with unlimited government funding and the ability to operate outside legal and moral obligations.
The facilities where the experiments take place and the marine battles the invaders are in Mars instead of its moons. Increased focus is placed on the importance of the UAC research being conducted (specifically research under the guidance of Malcolm Betruger) by a division called "Delta Labs". Like in the original games, a great deal of the UAC's research seems to be focused on teleportation, although experimentation with species retrieved from beyond the teleporters is also seen.
In Doom 3, the UAC on Mars was secretly aware of the existence of Hell even before the invasion. On the orders of Betruger, Delta Labs sent teams through the portals to capture demons for study.
In Doom 3 the UAC logo has a roughly triangular shape, with the acronym written near the bottom or base, although a logo fashioned after the original appears frequently on some machinery, and is seen exclusively in the older Mars caverns areas.
In the aftermath of the events of Doom 3 the UAC covers up the truth about demon invasion on Mars and blames the destruction of the facility on a mechanical accident. Then, two years after the events of Doom 3, they send another science team to Mars to investigate a mysterious transmission coming from the abandoned Site 1 dig site.
 The UAC in the Doom movie
The UAC has a role in the Doom movie as well, but their ambitions and scientific work deviate somewhat from those in the games, as they focus on genomic research, even though teleportation is also used as a form of transportation between Earth and Mars. It should be noted, however, that genomic research was mentioned in Doom 3. The genomic research in the movie focuses on a 24th chromosome which makes humans exceptionally strong and enduring and enhances or exacerbates their moral inclinations. In the movie the UAC operates on Mars and the brunt of the action takes place in the "Olduvai Research Station".
 UAC websites
For the promotion of Doom 3, id Software created a bogus UAC website. The "trajectory" mentioned on the site had two meanings. Within the context of the game, it represented the estimated arrival time to Mars of the shuttle that carried Swann, Campbell, and the player's Marine. In respect to the game as a product, it was actually a countdown to its release date. The site currently redirects to the id Software website (since early December 2005) but an archived copy can be accessed at the Internet Archive.
On the official website for Doom RPG, there are links to a Union Aerospace Corporation website, convincingly designed in the style of a large business company's home site. Many of the links lead to Access Denied pages to give an impression of confidentiality, but it shows some information on the fictional history of the UAC.
- The preliminary Doom Bible includes a fictitious military entity called the "UAAF" (which stands for "United Aerospace Armed Forces") that employs the player characters, although the UAC is also mentioned. The acronym "UAC" is not clarified, and is used primarily to mark a clean "Star Wars Imperial or 2001" style in the facilities built by an interstellar construction company named "StarStruct, Inc." Details of what specific role the UAC itself plays in the events are sketchy, however. The UAC and the UAAF operate on a fictional planet or moon called Tei Tenga, not on Mars and its moons.
- Official or custom levels depicting UAC facilities are often called "techbase" or "UAC base" levels by fans. The word "base" is commonly used for military facilities, and appears in the core Doom games in E1M9: Military Base from Doom (which incidentally does not display any UAC logos) and MAP10: Refueling Base from Doom II.
- A UAC "Dark Eye Armor Corps" is mentioned in some audio logs (Peter Raleigh; Han Lee) in Doom 3: it may be a reference by the Doom 3 writers to the 1995 game The Dark Eye, itself based on various macabre Edgar Allan Poe stories such as Berenice. The shared themes are necromancy, dementia and acts of self-mutilation similar to the observations of Dr. Peter Raleigh (Delta Sector 2a)