Current concepts of prevention are examined and a new approach offered. Prevention has different meanings and functions in the four major task areas of psychiatry: (1) medical tasks, (2) rehabilitative tasks, (3) social control tasks, and (4) humanistic tasks. Constructs of primary and secondary prevention are most useful in the medical task area. However, efforts at primary prevention of mental illness can have only limited effectiveness when we know so little about etiology. Secondary prevention is central to the medical caring tasks, where early diagnosis and treatment may lead to successful outcome. Tertiary prevention of disease and primary prevention of developmental defect are the work of the rehabilitative task area. The application of models of prevention in the social control and humanistic task areas has led to serious confusion.