Seventh-grade students (N = 324) completed social cognitive maps to identify peer groups and peer group leaders, sociometric nominations to describe their peers' behaviors, and questionnaires to assess their own behaviors. Peer group members resembled one another in levels of direct and indirect aggression and substance use; girls' cliques were more behaviorally homogenous than were boys' cliques. On average, leaders (especially if they were boys) were perceived as engaging in more problem behaviors than were nonleaders. In girls' cliques, peripheral group members were more similar to their group leader on indirect aggression than were girls who were more central to the clique. Peer leaders perceived themselves as being more able to influence peers but did not differ from nonleaders in their perceived susceptibility to peer influence. The findings contribute to our understanding of processes through which influence may occur in adolescent peer groups. (Contains 2 figures and 2 tables.)