Sir Derek Jacobi is a British actor, knighted in 1994 for his services to theatre, who has performed in three different media for Doctor Who.
His fist role was as the voice of the Master in the animated webcast, Scream of the Shalka. He would reprise the role in the 2007 television Utopia, where the Master was under the Chameleon Arch guise of "Professor Yana", and regenerated into John Simm's Master shortly after regaining his identity.
According to an interview he gave for Doctor Who Confidential, being on Doctor Who was a long term goal of his, ever since the 1960s, as was playing a part on Coronation Street. He has also played Ellis Peters' medieval-detective Brother Cadfael on ITV Television in the 1990s (interestingly, in season 1 the other main character of the series was played by Sean Pertwee, son of Jon Pertwee). Cadfael's role has also been played on radio by Philip Madoc and Glyn Houston.