UNIT is a fictional military organisation from the British science fiction television series Doctor Who and its spin-off series Torchwood and The Sarah Jane Adventures. 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).

Originally referred to as the United Nations Intelligence Taskforce, it was revealed in 2005 that the real-life UN was no longer happy being associated with the fictional organisation and UNIT's full name could now no longer be used (the "UNIT" and "UN" abbreviations could be used as long as it was not explained what the letters stood for). The organisation was renamed to the Unified Intelligence Taskforce in 2008, with the name first being used in the episode "The Sontaran Stratagem." Despite the series now distancing itself from the real-life UN, dialogue in the episode, and several since, indicates that the in-world fictional version of the United Nations still supports UNIT.

In a 2014 interview, former script editor Terrance Dicks recalled that he was present at the "birth" of UNIT during the production of the 1968 Doctor Who serial The Invasion. 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). In a series of interviews originally recorded for the 2006 DVD of the Doctor Who serial Inferno (1970), actor Nicholas Courtney, who played Brigadier Lethbridge-Stewart in The Invasion, similarly described The Invasion as a "dummy run" for the idea of the Doctor, the main protagonist of Doctor Who, being exiled to Earth. Dicks also said that the idea of exiling the Doctor was done because making every serial take place on Earth was cheaper to produce than if every serial had to have a new alien planet built, and that UNIT was an idea Sherwin had come up with to answer the question of what to do with the Doctor after he was exiled to Earth. Speaking in an interview on the 2012 DVD of the Doctor Who serial The Krotons (1968–69), Sherwin said that he wanted Doctor Who to be "down on Earth anyway, for credibility", and described UNIT as "the ideal vehicle" for this.

In another 2014 interview in Doctor Who Magazine, Sherwin recalled that after submitting his scripts for The Invasion to Bryant, which included UNIT, Sherwin, who was also working freelance as a script editor, was told by Bryant to introduce his UNIT idea earlier, as it could "take some of the weight off [the] shoulders" of actor Patrick Troughton, who played the Doctor. 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."

Sherwin said in 2014 that while working as script editor on the Doctor Who serial The Web of Fear (1968), which included several British Army characters, he told scriptwriters Mervyn Haisman and Henry Lincoln to include all of the characters that he had originally invented for The Invasion. Sherwin was uncertain if the military characters featured in The Web of Fear were part of UNIT, but was "convinced" that, as a teaser for UNIT's more substantial role in The Invasion, The Web of Fear was supposed to have replaced the military characters that were seen in the story. Sherwin asserted that he held the copyright on Lethbridge-Stewart, as he "created him in The Invasion". 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". Production notes in Doctor Who: The Complete History credit Haisman and Lincoln as the owners of Lethbridge-Stewart, who was the army commander from The Web of Fear, and mention how Bryant and director Douglas Camfield were negotiating the use of the character for The Invasion from Haisman and Lincoln in May 1968, subsequent to The Web of Fear being broadcast in February and March. The Web of Fear is also described in the notes as being a "major influence on The Invasion".

The roots of UNIT in the Doctor Who universe lie in the Second Doctor serial The Web of Fear (1968), following which the organisation is named and established in The Invasion (1968). 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).

UNIT continued to feature in Doctor Who after Spearhead, but when the Third Doctor's exile is lifted in The Three Doctors (1972–73), his association with UNIT becomes more sporadic, especially after his regeneration into his fourth incarnation at the end of Planet of the Spiders (1974). Although the last appearance of UNIT in the series for many years was in The Seeds of Doom (1976), the organisation continued to execute its mandate to investigate and combat alien activity. The final appearance of UNIT during the original run of Doctor Who was the Seventh Doctor serial Battlefield (1989).