The NATO Advanced Study Institute on the Prevention of Crime and Violence Among the Mentally III was held in May 1999 in Tuscany, Italy. Participants from 15 countries attended. Since care for persons with mental illness (schizophrenia, major depression, bipolar disorder, delusional disorder, atypical psychoses) has been deinstitutionalized, some persons with these disorders are committing crimes and serious violence. Consequently, societies around the world are confronted with a new challenge: to provide mental health care and social services to mentally ill persons in a humane way that will prevent illegal behaviours. Research in this field has been dominated by investigations designed to improve clinicians' accuracy in predicting violent behaviours, with little attention focused on the organization and implemen tation of treatments. The premise of the Advanced Study Institute was that treatments must have em pirically proven efficacy. Both professional ethics and public accountability require empirical evidence that each treatment will alleviate the problem that it targets. However, despite the fact that Western industrial societies provide treatment for mentally ill persons who have offended, there is a very limited base of knowledge on what constitutes effective treatment and how such treatments should be organized and delivered. The Advanced Study Institute was an attempt to stimulate and encour age research that will extend this knowledge base. The goals were to review what is known about mentally ill offenders and about effective treatments for them, and to provide a framework for the orientation of future investigations designed to improve treatment efficacy.