The socialization model views delinquent peers and beliefs as causally prior to delinquent behavior, whereas the selection model hypothesizes that association with delinquent peers and delinquent beliefs result from delinquent behavior. The interactional model combines aspects of both socialization and selection models, positing that these variables have bidirectional causal influences on one another over time. Data to test for reciprocal causality, drawn from three waves of the Rochester, New York, Youth Development Study, suggest that simple unidirectional models are inadequate. Associating with delinquent peers leads to increased delinquency via the peer network's reinforcing environment. Engaging in delinquency, in turn, leads to increased association with delinquent peers. In addition, delinquent beliefs exert lagged effects on peers and behavior which tend to harden the formation of delinquent beliefs. Appendixes contain supplemental data on delinquent peers, peer reactions, and delinquent beliefs. 60 references, 27 footnotes, 4 tables, and 2 figures