Peers play a crucial role in the development of children and adolescents, influencing a wide range of behaviors. Much emphasis has been put on peers as a risk factor; however, little is known about whether and how peers might protect against antisocial development or promote positive development. Recent methodological advances allow examination of peer processes (referring to influence and selection) in a statistically suitable way. However, influence and selection dynamics are encapsulated in contexts, such as classes and schools. Unfortunately, little is known about how contextual factors affect selection and influence processes. This proposal responds to this gap by examining to what extent contextual factors promote or hinder the development of antisocial behavior (bullying, aggression), prosocial behavior (defending, helping), and academic performance. This proposal offers an excellent opportunity to expand two databases of extraordinary quality for relatively low additional costs with four (KiVa) and six (SNARE) extra waves.