An important responsibility of emergency departments is the management of injuries sustained in assaults. Most assaults, including many causing serious injury, are not reported and not recorded by the police. This is important because police investigation and the conviction of offenders has a substantial deterrent effect and because information about the circumstances of intentional injury is key to prevention. Recent investigation of ED-police collaboration has shown that many of the injured, and ED staff want offenses to be reported but that there are attitudinal, logistic, and ethical-legal obstacles to achieving this. Organized joint efforts by emergency medicine personnel and police departments, on the basis of a sound legal and ethical framework to protect the rights of both victims and offenders, should deter more violent offenders and would-be violent offenders. They also provide the police with unique aggregate, nonconfidential information that is of substantial help in tackling violence. ED data can be used to measure and refine violence prevention initiatives and are being developed as the basis of a new, independent measure of police performance. Strategies, practical ideas to overcome obstacles, and directions for future research are suggested.