The authors discuss the development of the concept of prevention as it has evolved from the public health and mental health fields. Concepts of epidemiology, treatment, and community mental health are defined in terms of their contributions to the evolution of prevention thinking. Four models of prevention are presented and critiqued: the public health model, the operational model, the antecedent conditions model, and the injury control model. Essential ingredients for implementing effective preventive interventions are presented, as well as examples of practical preventive interventions.