This study was conducted to test whether group identification (importance of the group to the individual) covaried with individual-group similarity on problem behavior; and whether group identification moderated peer group influence on the individual’s development of delinquent behavior across a 1-year period. The level of reciprocated nominations within the individual’s self-nominated group was controlled for in all analyses. Participants were 190 sixth and seventh graders (during the first year of the study) from the north of Italy. Level of reciprocated nominations within the group, but not identification, was found to covary with individual-group behavioral similarity (group behavior interacted with reciprocity of group nominations in predicting individual behavior). Group identification, but not reciprocated nominations, was found to moderate peer group influence on the individual’s change in delinquent behavior, across 1 year. The individual’s peer status within the classroom, level of reciprocated nominations, and gender all were related to the individual’s level of group identification. Results are discussed in terms of understanding peer group influence on the individual.