Violence remains an important public safety and public health problem. Prior research has shown that denying firearm purchases by individuals who are at high risk for committing violent crimes, thereby restricting their access to firearms, is an effective violence prevention strategy. The Armed and Prohibited Persons System (APPS) is a new violence prevention initiative undertaken by the California Department of Justice (CalDOJ). APPS is used by law enforcement to systematically identify persons who, having legally purchased firearms in the past, have become prohibited persons in the present—usually as the result of a criminal conviction. Such individuals are at increased risk for future violence.