Twin studies of externalizing behavior problems in middle childhood and early adolescence suggest that there is moderate-to-substantial genetic variance and modest-to-moderate shared environmental variance in these behaviors. The present study is novel in three ways: it is a sibling adoption study, it employs both teacher and parent ratings of children's behaviors averaged over five assessments, and it explores aggression and delinquency separately. The sample included 78 adoptive sibling pairs and 94 biologically related sibling pairs in the Colorado Adoption Project. Parents and teachers completed ratings of the children's externalizing behavior problems at ages 7, 9, 10, 11, and 12 years. Boys and adopted children were rated as being somewhat higher in externalizing behavior problems. Sex differences in delinquency were more pronounced in adoptive than in nonadoptive families. Teachers' ratings showed evidence for moderate heritability and no shared environment for aggression and showed modest shared environment for delinquency. Parents' ratings showed evidence for moderate amounts of heritability and shared environment for both aggression and delinquency.