The authors investigated the relation between children's knowledge structures for peers and externalizing behavior problems. Initial levels of aggression were evaluated in 135 boys and 124 girls (Grades 1-3, 40% African American, 60% Caucasian) in Year 1 and again in Years 6 and 9. In Year 6, 3 aspects of their social knowledge structures were assessed: quality, density, and appropriateness. Results indicate that knowledge structures are related to children's concurrent levels of externalizing behaviors and that knowledge structures are related to children's concurrent levels of externalizing behaviors and predict externalizing behaviors 3 years later even after controlling for current levels of behavior. In addition, knowledge structures in Year 6 mediate the relation between aggression in Year 1 and externalizing behaviors in Year 9. The role of knowledge structures in the maintenance and growth of children's antisocial behavior is discussed.