This paper examines the contribution of peer relations to delinquency from the perspective of two sociological traditions: socialization/normative influence and opportunity. Earlier studies have likely overestimated normative influence by relying on respondents' reports about their friends' behaviors rather than obtaining independent assessments and by inadequately controlling for the tendency to select peers who are similar to oneself. Using detailed social network data from the National Longitudinal Study of Adolescent Health, we find support for both the socialization and opportunity models. Adolescents engage in higher rates of delinquency if they have highly delinquent friends and if they spend a great deal of time in unstructured socializing with friends. Yet our results also indicate that (1) the normative influence of peers on delinquency is more limited than indicated by most previous studies, (2) normative influence is not increased by being more closely attached to friends or spending more time with them, (3) the contribution of opportunity is independent from normative influence and of comparable importance, and (4) influences from the peer domain do not mediate the influences of age, gender, family or school.