Grounded in social ecological theory, bullying is understood as a group process that occurs in the presence of peers, and bystanders have a robust influence on bullying dynamics. Seminal work by Salmivalli and colleagues (1996) identified six participant roles: aggressors, targets, assistants, reinforcers, outsiders, and defenders. Assistants and reinforcers support bullying, outsiders remain passive, and defenders intervene to help victims. This framework has been widely applied in research on both traditional and cyberbullying. This chapter provides an overview of research on participant roles, including development and stability of roles over childhood and adolescence, and impact of the different participant roles on bullying victimization. The chapter highlights fluidity of these roles, as rigid classifications can oversimplify the complexities of peer dynamics in bullying. Finally, this chapter also reviews related characteristics (e.g., gender), skills (e.g., social emotional skills), and situational contexts associated with each participant role.