This study tests the contextual social-cognitive model, which has served as the basis for the Coping Power program, an indicated preventive intervention with at-risk preadolescent boys at the time of transition from elementary to middle school. The contextual social-cognitive model assumes that aggressive children have distortions in their social-cognitive appraisals and deficiencies in their social problem solving skills and that their parents have deficiencies in their parenting behaviors. To test this model, boys were identified as being at risk on the basis of fourth grade and fifth grade teachers' ratings of children's aggressive and disruptive behaviors, and interventions were delivered at the end of elementary school and the beginning of middle school. The intervention effect on delinquency, substance use, and school behavior outcomes was at least partially mediated through intervention-produced changes in child and parent variables that were targets for the intervention. These analyses provided unique support for the assumptions in the contextual social-cognitive model that changes in these mediating processes, even among high-risk boys, can have a meaningful impact on later negative outcomes.