Friends have been recognized as a potent source of influence on children's social, cognitive and emotional development. This chapter adopts a developmental perspective to understand whether and how friends or peers might influence children's/adolescents' conduct disorder. Four theoretical models regarding the role of deviant friends on the development of delinquency have been proposed: Peer influence, Individual characteristics, Social interactional, and a Mixed model. Given the relative lack of positive features in friendships with deviant youth, it is not surprising that these friendships do not buffer children against internalizing problems. Features of friendship can also act as moderators of deviant friends' influence on externalizing problems. Prevention/intervention studies are a good strategy to manipulate risk or protective factors in order to test their causal links with delinquency. Indeed, a study aimed at improving friends' characteristics or friendship quality and using an experimental design offers a real possibility to test causal links.