Sir Derek Jacobi is an English actor, best known to TV audiences for the title role in I, Claudius. Other notable screen roles include the title role in Cadfael, Sen. Gracchus in Gladiator, Alan Turing in Breaking the Code, Madson in Dead Again, and the narrator of In the Night Garden. He voiced the Master in the Doctor Who animated special Scream of the Shalka before guest-starring as the same character in the series proper in the episode "Utopia". He also appeared in an episode of Frasier as a thespian who, when Niles and Frasier were children, introduced them to the wonder of the theatre. When they met him again as adults and helped him get his own one-man show, he wasn't as good as they remembered. Many of the younger readers will know Derek Jacobi as Nicodemus from The Secret of NIMH.
He also has a distinguished stage career, having got his big break in Laurence Olivier's National Theatre company, including lots of Shakespeare; he has played Hamlet over 400 times on stage, not to mention his performance in the BBC's Complete Television Shakespeare. (He was also in Kenneth Branagh's film version of Hamlet, as Hamlet's villainous uncle -- another Claudius -- in addition to his role as Chorus in Branagh's Henry V.)
He was knighted for Services to Drama in 1994.