UFO is a 1970 British science fiction television series about the covert efforts of an international defence organisation (under the auspices of the United Nations) to prevent an alien invasion of Earth. It was created by Gerry Anderson and Sylvia Anderson with Reg Hill, and produced by the Andersons and Lew Grade's Century 21 for Grade's ITC Entertainment company.

A single series of 26 episodes (including the pilot) was filmed over the course of more than a year; a five-month production break was caused by the closure of MGM-British Studios in Borehamwood, where the show was initially made. Production then moved to Pinewood Studios in Buckinghamshire. UFO was first broadcast in the UK and Canada from 1970, and in the United States from 1972.

The Andersons' live-action science fiction movie Doppelgänger (also known as Journey to the Far Side of the Sun) is considered an immediate precursor to UFO, which was their first entirely live-action TV series. (Their previous shows had used marionettes.) The series featured actors, costumes, props, locations and music that had appeared in the film, and 11 cast members of the film appeared in at least one episode of UFO.

Following syndication in the US and initial favourable ratings, a possible second series was planned; initially entitled UFO 1999, this eventually became Space: 1999, but with a different cast from UFO.

The series' premise is that in 1980, Earth is being visited by aliens from a dying planet, who are abducting humans and harvesting their organs for their own bodies. The alien incursions may also be a prelude to a possible full-scale invasion. The series' main cast of characters are the staff of a secret, high-technology international military agency called SHADO (an acronym for Supreme Headquarters Alien Defence Organisation) established by the governments of the United Kingdom, the United States, the Soviet Union, France and Germany to defend Earth and humanity against the mysterious aliens and learn more about them, while at the same time keeping the threat of an alien invasion hidden from the public.

Operating under the cover (as well as literally beneath the premises) of the Harlington-Straker Studios movie studio in England, SHADO is headed by Commander Edward Straker (Ed Bishop), a former United States Air Force colonel and astronaut, whose "cover" is his role as the studio's chief executive.

SHADO has a variety of high-tech hardware and vehicles at its disposal to implement a layered defence of Earth. Early warnings of alien attack came from SID, the Space Intruder Detector, an unmanned computerised tracking satellite that constantly scans for UFO incursions. The forward line of defence is Moonbase from which the three lunar Interceptor spacecraft, that fire a single explosive warhead, are launched. The second line of defence includes Skydiver, a submarine mated with the submersible, undersea-launched Sky One interceptor aircraft, which attacks UFOs in Earth's atmosphere. The last line of defence is ground units including the armed, IFV-like SHADO Mobiles, fitted with caterpillar tracks.

On Earth, SHADO also uses two SHADAIR aircraft, a Seagull X-ray supersonic jet (e.g. in the episode "Identified") and a transport plane (e.g. in "A Question of Priorities"); a transatlantic Lunar Carrier with a separating Lunar Module (e.g. in "Computer Affair"); a helicopter (actually, a small VTOL aeroplane with large rotating propellers, e.g. in the episode "Ordeal"); and a radio-controlled Space Dumper (e.g. in "The Long Sleep"). The Moonbase has hovercraft-like Moon Hoppers/Moonmobiles that can be deployed for transportation or reconnaissance.