Thomas Mark Harmon is an American actor. He is perhaps best known for playing the lead role of Leroy Jethro Gibbs on NCIS. He has appeared in a wide variety of television roles since the early 1970s, including Dr. Robert Caldwell on St. Elsewhere, Detective Dicky Cobb on Reasonable Doubts, and Dr. Jack McNeil on Chicago Hope. He also starred in such films as Summer School, Prince of Bel Air, Stealing Home, Wyatt Earp, Fear and Loathing in Las Vegas, Freaky Friday, and Chasing Liberty.