Research on the prevalence, course, and correlates of behavior problems in preschool children was examined. Prospective epidemiological studies and follow-up studies of clinical/high risk samples indicate that serious externalizing problems identified early often persist. Negative, inconsistent parental behavior and high levels of family adversity are associated with the emergence of problems in early childhood and predict their persistence to school age. Studies are examined from a developmental perspective and integrated with research on optimal parent-child relationships. The severity of initial problems and family context are related to different developmental outcomes.