Despite the veritable influence of the peer context on the elaboration of adolescent aggression, few studies of relational aggression have directly identified and measured peer groups, limiting our ability to draw formal conclusions about the level and nature of peer influence. The current study used a developmental framework to examine peer group influence on individual levels of physical and relational aggression over a year with 6th and 7th grade students (n = 346, 51% female). A multilevel network approach was used in which peer groups were identified via social network analysis, and peer group influence was evaluated with hierarchical linear modeling (HLM). HLM analyses indicated that peer group aggression (physical and relational) at time 1 was predictive of individual aggression levels at time 2 after controlling for other peer group characteristics as well as other individual risk and protective factors. Although both forms of aggression were embedded in peer networks, findings suggest that physical aggression is relatively less endemic to peer networks, and is more likely to occur in smaller, predominantly male networks. The current study highlights the importance of understanding the influence of peer group membership on adolescent aggression and points to important implications for prevention.