The Black Panther Party (BPP), originally the Black Panther Party for Self-Defense, was a Marxist-Leninist Black Power political organization founded by college students Bobby Seale and Huey P. Newton in October 1966 in Oakland, California. The party was active in the United States between 1966 and 1982, with chapters in many major cities and international chapters in Britain and Algeria. Upon its inception the Black Panther Party's core practice was its open carry armed citizens' patrols ("copwatching") to monitor the behavior of officers of the Oakland Police Department and challenge police brutality in the city. From 1969 onwards, a variety of community social programs became a core activity. The Party instituted the Free Breakfast for Children Programs to address food injustice, and community health clinics for education and treatment of diseases including sickle cell anemia, tuberculosis, and later HIV/AIDS. It advocated for class struggle, with the party representing the proletarian vanguard.Black Panther Party members were involved in many fatal firefights with police. Newton declared:

Malcolm, implacable to the ultimate degree, held out to the Black masses ... liberation from the chains of the oppressor and the treacherous embrace of the endorsed [Black] spokesmen. Only with the gun were the black masses denied this victory. But they learned from Malcolm that with the gun, they can recapture their dreams and bring them into reality.

Huey Newton allegedly killed officer John Frey in 1967, and Eldridge Cleaver (Minister of Information) led an ambush in 1968 of Oakland police officers, in which two officers were wounded and Panther Bobby Hutton (Treasurer) was killed. FBI infiltrators caused the party to suffer many internal conflicts, resulting in the murders of Alex Rackley and Betty Van Patter.In 1967, the Mulford Act was passed by the California legislature and signed by governor Ronald Reagan. The bill was crafted in response to members of the Black Panther Party who were copwatching. The bill repealed a law that allowed the public carrying of loaded firearms.

In 1969, Federal Bureau of Investigation Director J. Edgar Hoover described the party as "the greatest threat to the internal security of the country." He made use of an extensive counterintelligence program (COINTELPRO) of surveillance, infiltration, perjury, police harassment, and many other tactics, designed to undermine Panther leadership, incriminate and assassinate party members, discredit and criminalize the Party, and drain organizational resources and manpower. The program was responsible for the assassination of Fred Hampton, and is accused of assassinating other Black Panther members, including Mark Clark.Government persecution initially contributed to the party's growth, as killings and arrests of Panthers increased its support among African Americans and the broad political left, who both valued the Panthers as a powerful force opposed to de facto segregation and the military draft. The party enrolled the most members and had the most influence in the Oakland-San Francisco Bay Area, New York, Chicago, Los Angeles, Seattle, and Philadelphia. There were active chapters in many prisons, at a time when an increasing number of young African-American men were being incarcerated.

Black Panther Party membership reached a peak in 1970, with offices in 68 cities and thousands of members, but it began to decline over the following decade. After its leaders and members were vilified by the mainstream press, public support for the party waned, and the group became more isolated. In-fighting among Party leadership, fomented largely by the FBI's COINTELPRO operation, led to expulsions and defections that decimated the membership. Popular support for the Party declined further after reports of the group's alleged criminal activities, such as drug dealing and extortion of Oakland merchants. By 1972 most Panther activity centered on the national headquarters and a school in Oakland, where the party continued to influence local politics. Though under constant police surveillance, the Chicago chapter also remained active and maintained their community programs until 1974. The Seattle chapter persisted longer than most, with a breakfast program and medical clinics that continued even after the chapter disbanded in 1977. The Party continued to dwindle throughout the 1970s, and by 1980 had just 27 members.The Party's history is controversial. Scholars have characterized the Black Panther Party as the most influential black movement organization of the late 1960s, and "the strongest link between the domestic Black Liberation Struggle and global opponents of American imperialism". Other commentators have described the Party as more criminal than political, characterized by "defiant posturing over substance".