K9, occasionally written K-9, is the name of several fictional robotic canines in the long-running British science fiction television series Doctor Who, first appearing in 1977. K9 has also been a central character in three of the series' television spin-offs: the one-off K-9 and Company (1981), The Sarah Jane Adventures (2007–2011) and K9 (2009–2010). Although not originally intended to be a recurring character in the series, K9 was kept in the show following his first appearance because he was expected to be popular with younger audiences. There have been at least four separate K9 units in the series, with the first two being companions of the Fourth Doctor. Voice actor John Leeson has provided the character's voice in most of his appearances, except during season 17 of Doctor Who, in which David Brierley temporarily did so. The character was created by Bob Baker and Dave Martin, to whom rights to the character still belong; consequently, Baker's spin-off series K9, which is not BBC-produced, could not directly reference events or characters from Doctor Who, though it attempted to be a part of that continuity.

Elisabeth Clara Heath-Sladen was an English actress. She became best known as Sarah Jane Smith in the British television series Doctor Who, appearing as a regular cast member from 1973 to 1976, alongside both Jon Pertwee and Tom Baker, and reprising the role many times in subsequent decades, both on Doctor Who and its spin-offs, K-9 and Company (1981) and The Sarah Jane Adventures (2007–2011).