Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a mysterious and eccentric alien, a Time Lord known as the Doctor, who travels through time and space in his time machine, the TARDIS, which normally appears from the exterior to be a blue 1950s British police box. With his companions, he explores time and space, faces a variety of foes and saves civilizations, helping people and righting wrongs.

The program is listed in Guinness World Records as the longest-running science fiction television show in the world, and as the "most successful" science fiction series of all time, in terms of its overall broadcast ratings, DVD and book sales, iTunes traffic, and "illegal downloads". It has been recognised for its imaginative stories, creative low-budget special effects during its original run, and pioneering use of electronic music (originally produced by the BBC Radiophonic Workshop). The show is a significant part of British popular culture in the United Kingdom, and elsewhere it has become a cult television favorite. The show has influenced generations of television professionals, many of whom grew up watching the series.

The program originally ran from 1963 to 1989. After an unsuccessful attempt to revive regular production with a backdoor pilot in the form of a 1996 television film, the program was relaunched in 2005, produced in-house by BBC Wales in Cardiff. The first series was produced by the BBC; series two and three had some development money contributed by the Canadian Broadcasting Corporation (CBC), which was credited as a co-producer.Doctor Who also spawned spin-offs in multiple media, including the current television programmes Torchwood and The Sarah Jane Adventures, the standalone K-9, and a single 1981 pilot episode of K-9 and Company.

The Doctor has been principally played by eleven actors. The transition from one actor to another is written into the plot of the show as regeneration, whereby the character of the Doctor takes on a new body and, to some extent, new personality. Although each portrayal is different, and on occasion the various incarnations have even met one another, they are all meant to be aspects of the same character. The Doctor is currently portrayed by Matt Smith, who took up the role after David Tennant's final appearance in an episode broadcast on 1 January 2010.