NCIS is an American police procedural drama television series, revolving around a fictional team of special agents from the Naval Criminal Investigative Service, which conducts criminal investigations involving the U.S. Navy and Marine Corps.

NCIS follows a fictional team of Naval Criminal Investigative Service Major Case Response Team (MCRT special agents headquartered at the Washington Navy Yard in Washington, D.C. It is described by the actors and producers (on special features on DVD releases in the United States) as being distinguished by its comedic elements, ensemble acting, and character-driven plots.

NCIS is the primary law enforcement and counterintelligence arm of the United States Department of the Navy, which includes the United States Marine Corps. NCIS investigates all major criminal offenses (felonies)—i.e., crimes punishable under the Uniform Code of Military Justice by confinement of more than one year—within the Department of the Navy. The MCRT is frequently assigned to high-profile cases such as the death of the U.S. president's military aide, a bomb situation on a U.S. Navy warship, the death of a celebrity on a reality show set on a US Marine Corps base, terrorist threats involving U.S Naval and Marine Corps weapons, personnel and/or installations, and kidnappings of Navy and Marine Corps personnel and/or their dependents.

The MCRT is led by Supervisory Special Agent Leroy Jethro Gibbs (Mark Harmon). Gibbs’ team is composed of Special Agent and Senior Field Agent Anthony "Tony" DiNozzo (Michael Weatherly), Special Agent Timothy McGee (Sean Murray), and Special Agent (former Mossad liaison officer) Ziva David (Cote de Pablo), who replaced Caitlin "Kate" Todd (Sasha Alexander) when she was shot and killed by rogue Mossad agent Ari Haswari (Rudolf Martin) at the end of season two. The team is assisted in their investigations by Chief Medical Examiner Donald "Ducky" Mallard (David McCallum), his assistant Jimmy Palmer (Brian Dietzen), who replaced Gerald Jackson (Pancho Demmings), and Forensic Specialist Abigail "Abby" Sciuto (Pauley Perrette).

It is revealed, through flashbacks, that the head of the MCRT before Gibbs was Special Agent Mike Franks (Muse Watson), who led the unit when it was part of the Naval Investigative Service (NIS), the predecessor agency of the NCIS. Franks recruited Gibbs shortly after Gibbs' retirement from the Marine Corps, eventually retiring himself some years later. After Franks' departure, Gibbs recruited DiNozzo from the Baltimore Police Department's Homicide Section. The two were briefly joined by Vivian Blackadder (Robyn Lively), whom Gibbs recruited from the FBI. In the second part of the NCIS pilot, Blackadder allowed her emotions to nearly derail an anti-terror operation in Spain. Gibbs is noticeably disappointed; Blackadder is not present in the series' first regular episode, replaced by Caitlin Todd, a Secret Service agent who joins Gibbs' team after resigning from the Secret Service. Todd is eventually killed in the line of duty in the season two finale, and is ultimately replaced by Ziva David, who from season three to season seven is the Mossad liaison officer who eventually gains American citizenship and becomes a full agent. McGee first appears as a Field Agent assigned to the Norfolk Field Office. He uses his computer skills to aid the MCRT in subsequent investigations through the rest of season one, until he is officially promoted with his own desk at the Navy Yard in the beginning of season two.

NCIS is currently led by Director Leon Vance (Rocky Carroll). The first director seen in the series, Thomas Morrow (Alan Dale), left after being promoted to Deputy Director of the Department of Homeland Security. Jenny Shepard (Lauren Holly) was appointed director after Morrow in the first episode of season three, the first NCIS director to be in the regular series cast list. After Shepard was killed in a shootout at the end of season five, Vance—who was assistant director of NCIS before her death—was seen as acting director after her death and was promoted to take her place.

In 2011, NCIS was voted America’s favorite television show. On February 1, 2013, CBS renewed NCIS for an 11th season.