AbstractTo understand the development of cyber aggression during adolescence, it is important to consider the temporal variability of its potential predictors. This study uses a four-wave survey to investigate how changes in peer norms, parental norms, and parental communication are associated with two-year trajectories of online peer aggression. The sample includes 1521 Swiss middle school students (M_age T1 = 11.54, SD = 0.40; 48% female). The results showed that over time a better parental communication quality and anti-aggression norms predicted lower rates and slower development of cyber aggression. Moreover, parental variables emerged as a quite stable deterrent of aggressive conduct. Although entrance into adolescence is characterized by the rise of peer influence, results from this study suggest that parents maintain an important protective role.