Doctor Who is a British science fiction television series broadcast by the BBC since 1963. The series depicts the adventures of a Time Lord called the Doctor, an extraterrestrial being who appears to be human. The Doctor explores the universe in a time-travelling space ship called the TARDIS. The TARDIS exterior appears as a blue British police box, which was a common sight in Britain in 1963 when the series first aired. With various companions, the Doctor combats foes, works to save civilisations, and helps people in need.

Beginning with William Hartnell, thirteen actors have headlined the series as the Doctor; in 2017, Jodie Whittaker became the first woman to officially play the role on television. The transition from one actor to another is written into the plot of the series with the concept of regeneration into a new incarnation, a plot device in which a Time Lord "transforms" into a new body when the current one is too badly harmed to heal normally. Each actor's portrayal is unique, but all represent stages in the life of the same character, and together, they form a single lifetime with a single narrative. The time-travelling feature of the plot means that different incarnations of the Doctor occasionally meet.

The series is a significant part of popular culture in Britain and elsewhere; it has gained a cult following. It has influenced generations of British television professionals, many of whom grew up watching the series. Fans of the series are sometimes referred to as Whovians. The series is listed in Guinness World Records as the longest-running science-fiction television series in the world, as well as the "most successful" science-fiction series of all time, based on its overall broadcast ratings, DVD and book sales, and iTunes traffic.

The series originally ran from 1963 to 1989. There was an unsuccessful attempt to revive regular production in 1996 with a backdoor pilot in the form of a television film titled Doctor Who. The series was relaunched in 2005 and since then, has been produced in-house by BBC Wales in Cardiff. Doctor Who has also spawned numerous spin-offs, including comic books, films, novels, audio dramas, and the television series Torchwood (2006–2011), The Sarah Jane Adventures (2007–2011), K9 (2009–2010), and Class (2016). It has been the subject of many parodies and references in popular culture.