Over the last few decades, research on the role of classroom peer ecologies in student-to-student bullying has widened our understanding of this phenomenon. Bullying functions not only for individual perpetrators but also for the whole peer group by, for instance, providing a common goal and a semblance of cohesion for the group members. Bullying is more likely in classrooms characterized by poor climate, strong status hierarchy, and probullying norms. Bystanders' responses contribute to the bullying dynamic by either rewarding or sanctioning the behavior of the perpetrators. Bystanders' responses to bullying can be changed through school-based programs, mediating the effects of the programs on bullying. Further efforts are needed to transform evidence into schools' preventive practices encompassing the peer ecologies.