One line of evidence consists of findings that the onset of antisocial behavior in one sibling increases the risk to other siblings. A second line of evidence shows that the tendency of monozygotic twins to be more concordant for antisocial behavior than dizygotic twins can be explained by contagion as well as heredity. In addition, there are differences in prevalence between same-sex and opposite-sex twins that contagion can explain but heredity cannot, at least not without numerous ad hoc auxiliary suppositions. Behavioral contagion is also able to explain, and very precisely, the temporal course of aggregate delinquency through adolescence. Evidence has been presented that antisocial behavior is an equilibrium phenomenon; that is, it depends on a balance between antisocial and prosocial forces, a balance or imbalance that would explain sudden leaps and falls that crime statistics sometimes take. Finally, programs to combat drug use are often modestly successful and almost all such programs presuppose that drug use is contagious. The article closes by discussing the implications of the different lines of evidence for policy and practice. 65 references, 3 tables, and 2 figures