Although originating in California, prison gangs are now a nationwide problem. A prison gang is an organization which operates within the prison system as a self perpetuating, criminally-oriented enterprise, consisting of a select group of inmates who have established an organized chain of command and are governed by a code of conduct. The prison gang will usually operate in secrecy and conducts activities by controlling the prison environment through intimidation and violence toward nonmembers. Prison gangs typically have some form of initiation process for recruits to prove their worthiness for membership in the organization. The five major prison gangs are the Mexican Mafia, the Neustra Familia, the Aryan Brotherhood, the Texas Syndicate, and the Black Guerilla Family. Drug trafficking is the primary criminal enterprise of these families, although they also engage in extortion and robbery. The chapter provides illustrations of gang tattoos and outlines codes of conduct for each gang.