Operating under the auspices of the United Nations and initially led by Brigadier Lethbridge-Stewart, its purpose is to investigate and combat paranormal and extraterrestrial threats to Earth.

Several UNIT personnel (such as the Brigadier, Sergeant Benton and Mike Yates) played a major role in the original Doctor Who series, and it was a regular feature from The Invasion (1968) until The Seeds of Doom (1976).

He credited writer and script editor Derrick Sherwin and producer Peter Bryant as having come up with the idea beforehand, saying that they were testing the concept in The Invasion before it had become central to the show in the Doctor Who serial Spearhead from Space (1970).

[8] Speaking in an interview on the 2011 special-edition DVD of Spearhead from Space, Sherwin said that he had created UNIT because he wanted to give some "considerable support" to the Doctor, "so that [Troughton] didn't have so many damn lines to learn each week".

[10] In an interview recorded for the 2006 DVD release of Inferno, Sherwin described The Invasion as the start of UNIT and the beginning of the Doctor "coming down to Earth".

[14] According to "Survivors of the Flux" (2021), UNIT was founded in 1958 and built over the subsequent decade, with Lethbridge-Stewart joining the taskforce after it failed to act on the events of the First Doctor story The War Machines (1966).

Following The Invasion, the contribution of scientific advice in battling extraterrestrial threats is recognised and both Dr Elizabeth Shaw and the exiled Third Doctor joins UNIT just in time to help defeat the Autons in Spearhead from Space (1970).

[17] Although the last appearance of UNIT in the series for many years was in The Seeds of Doom (1976),[18] the organisation continued to execute its mandate to investigate and combat alien activity.

[19] UNIT is mentioned by both its acronym and full name in the 2005 series episodes "Aliens of London" and "World War Three", where it sent a delegation to a gathering of experts at 10 Downing Street.

UNIT is resurrected in the 2022 BBC Centenary special "The Power of the Doctor", once again under the leadership of Kate Stewart with a new building in London which is demolished by the end of the episode to defeat the Cybermen.

Due to the international nature of the organisation, it is sometimes viewed with suspicion by local military and national security agencies, who feel that it might impinge on their sovereignty.

[36] A mobile command centre is also shown in "The Sontaran Stratagem" and "The Poison Sky", where it is depicted as a black articulated lorry with UNIT insignia.

[22] In "The Poison Sky", UNIT is shown to be able to command and co-ordinate the world's combined nuclear arsenal for strategic strikes on orbiting alien craft.

[40] The Valiant is also shown (in "The Poison Sky") to be equipped with a scaled down version of the Torchwood Institute weapon that destroyed the Sycorax ship in "The Christmas Invasion".

[25] "The Day of the Doctor" (2013) shows that the Tower of London base contains the Black Archive housing various alien technological devices that UNIT has salvaged over the years and kept hidden away.

UNIT is also shown to at least have access to the Under-Gallery, a secret gallery containing works of art dating back to the Elizabethan era that Elizabeth I ordered locked away as they were too dangerous for the public.

In 1984, a stage comedy titled Recall UNIT: The Great T-Bag Mystery was produced, written by Richard Franklin (Captain Yates) who reprised his character in the play.

[63] Due to other commitments, Nicholas Courtney was unable to appear as the Brigadier, but pre-recorded a telephone message from Lethbridge-Stewart which was written into the plot.

Broadsword agents wore plain clothes and were "hand-picked to offer us lateral and non-military solutions, backed up by SAS training and sheer common sense".

[67] By the 26th century, UNIT has transformed into a secret society called the Unitatus, pledged to defend the Earth against alien threats, first seen in Parkin's Cold Fusion (1996).

[77] The Tenth Doctor comic strip The Age of Ice (DWM #408–411) is set in UNIT's Australian base beneath Sydney Harbour.

218 of Uncanny X-Men, a character identified as Brigadier Lethbridge-Stewart is seen briefly from behind, addressing a Sergeant-Major Benton who is loading an unconscious Juggernaut into a lorry; 2000 AD's Caballistics, Inc. strip has Lethbridge-Stewart (referred to solely by rank) appearing in several adventures as a military liaison and referring to The Web of Fear; and Hip Flask has a 22nd-century UNIT tied into the origins of the Elephantmen.

The storyline concludes with UNIT nurse Tara Misha joining the TARDIS crew after she sacrifices her own reputation to expose Avalon's deceptions.

An alternate universe version of UNIT and the Brigadier appear in the Doctor Who Unbound audio play Sympathy for the Devil (2003), produced by Big Finish Productions.

The story concerns a UNIT that never had the Third Doctor working for it, with many different outcomes; Terror of the Autons resulted in "the Plastic Purges", Mike Yates died on a time-travel mission to destroy the Silurians, and so on.

[97] For the new television series, BBC created a faux website for UNIT,[98] complete with "easter eggs" that can be accessed by the reader with the passwords "bison" and "buffalo" (the latter mentioned on screen in "World War Three").

The 'public' part of the website advertises UNIT Conferences and publications relating to "extra-territorial threats", as well as press releases on the establishment of a central New York Liaison office; the press releases and publications also make reference to off-screen adventures, such as the Skaniska Incident and Jersey Tollgate Situation, with the most recent covering the events of "The Christmas Invasion" (2005) ("Alien Life Confirmed").

The Secure Login link, using the password "badwolf" (originally "bison") uncovers a 'private' section which provides UNIT point-of-view reports about various events in the 2005 series, as well as mention of missions such as The Fourth Reich and Guatemala "Big Locust" Problem.

The original 1963–1989 series presents conflicting evidence about when the stories featuring UNIT are meant to take place, and there has been much confusion and continuing fan debate on this subject.

When the crisis concludes after the Seventh Doctor brings the two aspects together, he notes that the resulting temporal explosion will cause a range of minor anomalies to anyone in this era who has dealt with time travel, reflecting that this explains how someone who was working in the eighties could have retired in the seventies.