Timing and duration associated with the course of antisocial involvement are the defining features of the two types of offenders. In contrast to the small group of life course persistent antisocial persons, a larger group of persons have criminal careers of shorter duration. For offenders whose criminal activity is confined to the adolescent years, causal factors may be proximal, specific to the period of adolescent development, and theory must account for discontinuity in their lives. In contrast, for persons whose adolescent delinquency is merely one inflection in a continuous antisocial course, antisocial behavior theory must locate causal factors early in childhood and must explain the continuity in their troubled lives. An etiological theory for persistent antisocial behavior over the life course is described, with emphasis on social interaction and psychopathology. Likewise, an etiological theory for adolescence-limited antisocial behavior is presented that focuses on motivation and reinforcement. Research strategies are detailed to guide epidemiological predictions about delinquency. 154 references