The findings suggest a curvilinear association between aggression and changes in the quality of boys' friendships, but a linear association for girls. Therefore, the negative association between aggression and friendship quality over time was constant across levels of aggression for girls. In contrast, the association between aggression and friendship quality for boys varied as a function of their level of reported aggression. The finding that nonaggressive behavior is linked to high friendship quality over time supports previous research. The findings emphasize the need to look beyond linear effects in the study of adolescent social development and suggest that there may be qualitative differences between boys and girls in how aggression is related to friendship quality during early adolescence. This study builds on the literature linking aggression, popularity, and social skills to investigate how aggression is related to adolescents' perceptions of quality of friendships, which are based on trust, communication, and closeness. Specifically, in a sample of 246 boys and 253 girls, the study investigated linear and curvilinear associations between overt aggressive behavior and the adolescents' reports of the quality of their friendships over time. Tables, figure, and references