The articles in this volume illustrate how development is propelled by the bidirectional relations that occur between the person and all levels of the context. The authors argue that adolescent life is embedded in a complicated developmental system involving multiple features of the individual (e.g., biology, emotions, personality, and cognition) and the multiple levels of his or her social ecology (e.g., peers, family, school, the workplace, and the public policy and legal systems that structure and impact behavioural opportunities for and the actions of adolescents). These articles have important implications for the design of interventions aimed at adolescent problem behaviours.