This study, drawing on approximately 1,100 males from the National Longitudinal Study of Adolescent Health, demonstrates the importance of genetics, and genetic-environmental interactions, for understanding adolescent delinquency and violence. Our analyses show that three genetic polymorphisms--specifically, the 30-bp promoter-region variable number tandem repeat (VNTR) in MAOA, the 40-bp VNTR in DAT1, and the Taq1 polymorphism in DRD2--are significant predictors of serious and violent delinquency when added to a social-control model of delinquency. Importantly, findings also show that the genetic effects of DRD2 and MAOA are conditional and interact with family processes, school processes, and friendship networks. These results, which are among the first that link molecular genetic variants to delinquency, significantly expand our understanding of delinquent and violent behavior, and they highlight the need to simultaneously consider their social and genetic origins. (Contains 6 footnotes and 8 tables.)