This book examines the role of peer relationships in child and adolescent development by tracking research findings from the early 1900s to the present. Dividing the research into three generations, the book describes what has been learned about children's peer relations and how children's participation in peer relationships contributes to their health, adjustment, and achievement. Gary W. Ladd reviews and interprets the investigative focus and findings of distinct research eras to highlight theoretical or empirical breakthroughs in the study of children's peer relations and social competence over the last century. He also discusses how this information is relevant to understanding and promoting children's health and development. In a final chapter, the author appraises the major discoveries that have emerged during the three research generations and analyzes recent scientific agendas and discoveries in the peer relations discipline.