James Callis appears as Dr. Gaius Baltar in SCI FI Channel's series Battlestar Gallactica, opposite Edward James Olmos and Mary McDonnell, and was also seen in the USA Network miniseries Helen of Troy.

The London-born Callis studied English and Related Literature at the University of York. After graduating in 1993, he gained a place at the renowned London Academy of Music & Dramatic Arts, from which he graduated in 1996. Since then, he has appeared in various West End stage productions as well in film and on television and radio. He made his West End debut in Old Wicked Songs alongside Bob Hoskins in 1996, earning the London Critics' Circle's Jack Tinker Award for Most Promising Newcomer. Other theater roles include The Doctor's Dilemma at the Almeida in London and Things You Shouldn't Say Past Midnight at the Soho Theatre in London.

He broke into television in 1996 with a guest role on the British series Murder Most Horrid and with a recurring role in the long-running U.K. drama Soldier, Soldier. Callis went on to appear in a number of films and miniseries, including The Scarlet Pimpernel, Jason and the Argonauts and first played Bridget's pal Tom in Bridget Jones's Diary. He also co-wrote and co-directed the film Beginner's Luck, in which he starred opposite Julie Delpy.