In this article, longitudinal social network data are analyzed to get a better understanding of the interplay between delinquent peers and delinquent behavior. These data contain detailed information about the social networks of secondary school students from the same grade, their delinquent behavior, and many relevant correlates of network formation and delinquency. To distinguish selection and influence processes, a method (Simulation Investigation for Empirical Network Analyses, SIENA) is used in which network formation and changes in delinquency are simulated simultaneously within the context of other network processes and correlates of delinquency. The data and the method used make it possible to investigate an unusually wide array of effects on peer selection and delinquent behavior. The results indicate that similarity in delinquency has no significant effect on the selection of school friends when other network dynamics are taken into account. However, the average delinquency level of someone's friends in the school network does have a significant, although relatively small, effect on delinquent behavior of the respondents, beyond significant effects of changes in the level of self-control and morality. Another peer-related change, leaving or joining informal street-oriented youth groups, also appears to have a substantial effect on changes in delinquency. (Published Abstract) Tables and references