This chapter describes why social networks of peers become increasingly important in adolescence, and to which extent peer selection and influence processes take place in relation to adolescents’ risk behaviors, internalizing problems, and adaptive behaviors. Different types of selection (e.g., default selection, preferential attraction) and influence processes (e.g., peer pressure, imitation) are explained. The chapter provides an overview of individual, dyadic, and contextual factors contributing to variations between adolescents in their openness to peer influence and ends with directions for further research.