Three theoretical models of the interrelations among associations with delinquent peers, delinquent beliefs, and delinquent behavior are examined. The socialization model views delinquent peers and beliefs as causally prior to delinquent behavior, whereas the selection model hypothesizes that associations with delinquent peers and delinquent beliefs are a result of delinquent behavior. The interactional model combines aspects of both the socialization and the selection models, positing that these variables have bidirectional causal influences on one another over time. Data to test for reciprocal causality are drawn from three waves of the Rochester Youth Development Study. Results suggest that simple unidirectional models are inadequate. Associating with delinquent peers leads to increases in delinquency via the reinforcing environment of the peer network. Engaging in delinquency, in turn, leads to increases in associations with delinquent peers. Finally, delinquent beliefs exert lagged effects on peers and behavior, which tend in turn to “harden” the formation of delinquent beli