Prison gangs are often thought to create a “culture of violence” in U.S. prisons and jails. Yet mounting research and evidence suggests that prison gangs, in pursuit of profits from illicit market activity, also act as a check on the violent behavior of the broader prison population. This paper synthesizes existing research on prison gangs into a modeling framework that treats gangs as profit-maximizing suppliers and sources of informal governance in an illicit marketplace. The model offers broad policy implications that highlight the challenges and potential unintended consequences of correctional policies that address violence and gang activity.