Doctor Who is a British science fiction television (science fiction on television) programme produced by the British Broadcasting Corporation (BBC) (BBC). The programme depicts the adventures of a mysterious and eccentric humanoid alien known as the Doctor (Doctor (Doctor Who)) who travels through time (time travel) and space in his spacecraft, the TARDIS (an acronym for Time And Relative Dimensions In Space), which normally appears from the exterior to be a blue 1950s British police box. With his companions (Companion (Doctor Who)), he explores time and space, faces a variety of foes (List of Doctor Who monsters and aliens) and rights wrongs.

The programme 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 (Culture of the United Kingdom); in the United Kingdom and elsewhere, it has become a cult television (cult following) favourite and has influenced generations of British television professionals, many of whom grew up watching the series. It has received recognition (#Awards) from critics and the public as one of the finest British television programmes, including the BAFTA Award (British Academy Television Awards) for Best Drama Series (British Academy Television Award for Best Drama Series) in 2006, and five consecutive wins at the National Television Awards since 2005, in the Drama category.

The programme 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 (Doctor Who (1996 film)), the programme was relaunched in 2005 (History of Doctor Who#The 2000s), produced in-house by BBC Wales in Cardiff. The first was produced by the BBC and series two and three of the new series had some development money contributed by the Canadian Broadcasting Corporation (CBC), which was credited as a co-producer. Doctor Who also spawned spin-offs (Doctor Who spin-offs) in multiple media, including the current television programmes Torchwood and The Sarah Jane Adventures, the standalone K-9 (K-9 (TV series)) and a single 1981 pilot episode of K-9 and Company.

The Doctor has been played by eleven actors. The transition from one actor to another is written into the plot of the show as regeneration (Regeneration (Doctor Who)), 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 encountered one another, they are all meant to be aspects of the same character. The Doctor is currently portrayed by Matt Smith (Matt Smith (actor)), who took up the role after David Tennants final appearance in an episode broadcast on 1 January 2010. A fifth series of the relaunched programme began on 3 April 2010, in which the Eleventh Doctor is accompanied by Amy Pond, portrayed by Karen Gillan.