NCIS is an American television series combining elements of police procedural and military genres centered on a team of agents from the Naval Criminal Investigative Service (NCIS), a federal agency committed to investigating crimes involving branches of the U.S. military.

Leading the Major Case Response Team (MCRT) is Special Agent Leroy Jethro Gibbs, a former Marine sniper with a deep loyalty towards the agents on his team and considers them his adopted family, which currently includes Tim McGee, Ellie Bishop, Nick Torres, Abby Sciuto, NCIS Director Leon Vance, Dr. Donald "Duckie" Mallard, Jimmy Palmer, NCIS Senior Agent Jacqueline "Jack" Sloane and MI6 agent Clayton Reeves.

Former members of the MCRT include Kate Todd, former director Jenny Shepard, Ziva David, Tony DiNozzo and Alex Quinn.

The concept and characters of NCIS were initially introduced in two episodes of the CBS series JAG (season eight episodes "Ice Queen" and "Meltdown"). The show, itself a spin-off from JAG, premiered on September 23, 2003, on CBS. To date, it has aired for fourteen full seasons. Season Fifteen will premiere on Tuesday September 26, 2017 at 8/7c.

The series has produced two spin-offs; NCIS: Los Angeles in 2009 and NCIS: New Orleans in 2014.