"Slitheen" was a piece of instrumental music composed by Murray Gold as the main leitmotif for the Slitheen family.
It appeared throughout the series 1 episodes Aliens of London, World War Three, and Boom Town.
It reappeared in Love & Monsters, during Elton's flashback to the events of Aliens of London, when the Slitheen ship crashes into Big Ben.
Its final appearance to date was in the opening of The Day of the Doctor, as the TARDIS is being taken to the Tower of London.