The Master, or "Missy" (short for "Mistress") in their female incarnation, is a recurring character and one of the main antagonists of the British science fiction television series Doctor Who and its associated spin-off works.

Multiple actors have played the Master since the character's introduction in 1971. Within the show's narrative, the change in actors and subsequent change of the character's appearance is sometimes explained as the Master taking possession of other characters' bodies or as a consequence of regeneration, which is a biological attribute that allows Time Lords to survive fatal injuries or old age.

The Master was originally played by Roger Delgado from 1971 until his death in 1973. The role was subsequently played by Peter Pratt, Geoffrey Beevers, and Anthony Ainley, with Ainley reprising the role regularly through the 1980s until the series’s cancellation in 1989. Eric Roberts took on the role for the 1996 Doctor Who TV film. Since the show's revival in 2005, the Master has been portrayed by Derek Jacobi, John Simm, Michelle Gomez, and Sacha Dhawan.

Beevers, Roberts, Jacobi, Simm, Gomez, and Dhawan have reprised the role in audio dramas produced by Big Finish Productions, while Alex Macqueen, Gina McKee, Mark Gatiss, James Dreyfus, and Milo Parker have portrayed incarnations unique to Big Finish.

The creative team conceived of the Master as a recurring villain, first appearing in Terror of the Autons (1971). The Master's title was deliberately chosen by producer Barry Letts and script editor Terrance Dicks because, like the Doctor, it was a title conferred by an academic degree. A sketch of three "new characters" for 1971 (the other two being Jo Grant and Mike Yates) suggested he was conceived to be of "equal, perhaps even superior rank, to the Doctor."

Letts only had one man in mind for the role: Roger Delgado, who had a long history of playing villains and had already made three attempts to be cast in the series. He had worked previously with Letts and was a good friend of Jon Pertwee.

Malcolm Hulke spoke of the character and his relationship with the Doctor: "There was a peculiar relationship between the Master and the Doctor: one felt that the Master wouldn't really have liked to eliminate the Doctor...you see the Doctor was the only person like him at the time in the whole universe, a renegade Time Lord and in a funny sort of way they were partners in crime."

An unrelated character also known as the Master, who ruled over the Land of Fiction, had previously appeared in the 1968 serial The Mind Robber opposite the Second Doctor.