||This article relies largely or entirely upon a single source. (April 2013)|
A Prison gang is any type of gang activity in prisons and correctional facilities. Prison officials and others in law enforcement use the term security threat group or STG. The concept for this name is to take away the recognition and publicity that the term "gang" connotes when referring to people who have an interest in undermining the system.
Most prison gangs do more than offer simple protection for their members. Most often, prison gangs are responsible for any drug, tobacco or alcohol handling inside correctional facilities. Furthermore, many prison gangs involve themselves in prostitution, assaults, kidnappings and murders. Prison gangs often seek to intimidate other inmates (pressuring them to relinquish their food and other resources) and bribe or intimidate prison staff (to ensure they can go about their activities without interference, and to create links to the outside).
In addition, prison gangs often exercise a large degree of influence over organized crime in the "free world", larger than their isolation in prison might lead one to expect. Since the 1980s, larger prison gangs have consciously worked to leverage their influence inside prison systems to control and profit from drug trafficking on the street. This is made possible based upon the logic that individuals involved in selling illegal drugs face a high likelihood of serving a prison term at some point or in having a friend or family member in prison.
The cooperation of drug dealers and other criminals can be secured due to the credible threat of violence upon incarceration if it is not provided. Prison gang members and associates who are released are usually expected to further the gang's activities after their release and may face danger if they refuse and are returned to prison, such as on a parole violation. The War on Drugs also led to large numbers of drug addicts serving prison terms, providing gangs with a significant method of asserting control within the prison and by controlling the drug trade that happens on the yard and behind bars.
Prison gangs can also be responsible for laundering money from outside gangs, usually the free world branches of the same gangs "on the inside."
Most correctional facilities have policies prohibiting the formation of prison gangs. However, many prison gangs continue to operate with impunity. As these gang members are already in prison, and often serving long sentences, any punishment incentive to leave a gang or to integrate with the general prison population is reduced.
Prison gangs often have several "affiliates" or "chapters" in different state prison systems that branch out due to the movement or transfer of their members. Smaller prison gangs may associate with or declare allegiance to larger ones. In addition, some prison gang "chapters" may split into antagonistic groups that become rivals, as the Mexican Mafia did in Arizona (into the "Old" or "Original" Mexican Mafia associated with the original California gang and the "New Mexican Mafia", a rival group).
In the United States 
In other countries 
- The Numbers Gang, South Africa
- Primeiro Comando da Capital, Brazil
- Lynx gang, United Kingdom
- Brödraskapet, Sweden
- Hagedorn, John M. (2008), A World of Gangs: Armed Young Men and Gangsta Culture, Minneapolis, Minnesota, United States: University of Minnesota Press, ISBN 978-0-8166-5066-8