Thomas Mark Harmon is an American actor and former football player. 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.

Leroy Jethro Gibbs is a fictional character and the original protagonist of the CBS TV series NCIS, portrayed by Mark Harmon. He is a former U.S. Marine Corps Scout Sniper turned special agent who commands a team for the Naval Criminal Investigative Service. Gibbs is the most accomplished marksman on the team and the most skilled at handling violent standoffs; he depends on his other agents heavily for technical forensics and background checks. He is patient but firm with his team and has little patience for bureaucracy; he commands most other main characters—including his current staff Timothy McGee, Nick Torres and, briefly, Jessica Knight and previous staff Caitlin Todd, Anthony DiNozzo, Ziva David, Alexandra Quinn, Clayton Reeves, Ellie Bishop and Jacqueline Sloane.