Evidence is presented showing that aggression is functional. The reinforcers provided vary as a function of age and setting. During the toddler years, negative reinforcing contingencies supplied by the caretaker and family members control the occurrence of overt forms of antisocial behavior. Beginning during school years, positive reinforcers supplied by members of the deviant peer group shape and control the occurrence of covert forms. Boys who move through both of these developmental stages are at risk for early police arrest, and in turn for chronic juvenile and adult offending. Studies in the early 1970s showed that training parents to alter these contingencies effectively reduced rates of antisocial behavior. These behavioral approaches have been tested in numerous studies that employ randomized trials, objective measures and follow-up designs. The intervention components have also been extended for use by foster parents trained and supervised in the use of these procedures with chronic offending adolescents. Prior to the adolescents return to their homes, the biological parents are also trained and supervised. Follow-up data show significant reductions in police arrest and rates of institutionalization. Taken together, these findings clearly support the efficacy of behavioral strategies in constructing etiologic models and a set of strategies for effective intervention with antisocial behaviors.