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. ********** In many respects aggressive behavior is ideally suited to study by behavioral procedures. Antisocial behavior is readily observable in most settings. Observers can be trained to be highly reliable in coding such behavior (Jones, Reid, & Patterson, 1975). It is also a facet of behavior that is highly stable over time. For example, Olweus (1979) showed measures of antisocial behavior to be at least as stable over time as are measures of intelligence. There is modest support for the finding that measures obtained during preschool years are significant predictors for measures obtained as young adults (Kagan & Moss, 1962; Stattin & Magnusson, 1991). Several studies have shown that, given preschool identification as antisocial, then the odds are about 50% to 60% of being so classified as adolescents (Kazdin, Mazurick, & Bass, 1993; Tremblay, Boulerice, Pihl, Vitaro, & Zoccolillo, 1996). One interesting finding from studies such as these is that during late childhood, there are only very small numbers of new cases added. It seems then that most chronic antisocial individuals begin the trajectory during preschool years (Fergusson, Horwood, & Lynskey, 1995; Nagin & Tremblay, 1999; Patterson, Shaw, Snyder, & Yoerger, 2001). Epidemiologists estimate that about 8% of boys and fewer than 3% of girls might fit the definition of demonstrating early emerging and persisting extreme antisocial behavior (Offord, Boyle, & Racine, 1991). There is also an interconnectedness among the different forms of antisocial behavior. Children who engage in high rates of noncompliance are at significant risk for also engaging in higher rates of hitting, fighting, and stealing (Patterson, Reid, & Dishion, 1992). Adolescents who engage in high rates of trivial delinquent acts (e.g., petty theft) also tend to engage in high rates of violent delinquent acts (Capaldi & Patterson, 1996). This suggests that although each antisocial behavior may be maintained by its own set of contingencies, there may be some general sense in which they are all part of the same system. Childhood forms of antisocial behavior have been shown to be significant predictors for lifetime failures in achievement as shown in the pioneering longitudinal study by Huesmann, Eron, and Yarmel (1987). In keeping with this assumption, Olweus (1983) studied a large sample of Norwegian boys and found a significant path from antisocial behavior to poor grades. In an at-risk Oregon sample, a structural equation model showed a path coefficient of -. …