The presumed link between mental disorder and violence has been the driving force behind mental health law and policy for centuries. Legislatures, courts, and the public have come to expect that mental health professionals will protect them from violent acts by persons with mental disorders. Yet for three decades research has shown that clinicians unaided assessments of dangerousness are barely better than chance. Rethinking Risk Assessment: The MacArthur Study of Mental Disorder and Violence tells the story of a pioneering investigation that challenges preconceptions about the frequency and nature of violence among persons with mental disorders, and suggests an innovative approach to predicting its occurrence. The authors of this massive project -- the largest ever undertaken on the topic -- demonstrate how clinicians can use a decision tree to identify groups of patients at very low and very high risk for violence. This dramatic new finding, and its implications for the every day clinical practice of risk assessment and risk management, is thoroughly described in this remarkable and long-anticipated volume. Taken to heart, its message will change the way clinicians, judges, and others who must deal with persons who are mentally ill and may be violent will do their work.