This text examines the cognitive, emotional, and biological changes going on within the adolescent as he or she interacts with peers on the road to adulthood. The peer relationship is shown to be the most influential force in this period of development. The author presents a new theory--based on empirical data from research with 2,500 adolescents--that makes it possible to identify stages of adolescent development and reinterpret the importance of the peer group in the development of self-concept. She also discusses practical therapeutic approaches.