This article focuses on two research questions. To what degree is similarity in delinquent behaviour among Dutch secondary school students explained by selection and influence processes? And to what extent is the effect of friends’ delinquency on adolescents’ delinquency level moderated by characteristics of the friendship relation? Hypotheses were drawn from Sutherland’s Differential Association Theory, its extension by Burgess and Akers, and Hirschi’s Social Control Theory. The hypotheses were tested with longitudinal social network data, using the recently developed software program ‘Simulation Investigation for Empirical Network Analysis’ (SIENA). The network data were gathered among lower educated students of twelve secondary schools in the Dutch province of South Holland during a three-year period. The analyses suggest that adolescents tend to adjust their behaviour to the delinquency level of their peers (influence process), and tend to be friends with others who have a similar level of delinquency (selection process). Further, the results indicate that attachment to friends, time spent with friends and social pressure of friends do not affect the influence of peers on adolescents’ delinquency level.