Book excerpt: This comprehensive, authoritative handbook covers the breadth of theories, methods, and empirically based findings on the ways in which children and adolescents contribute to one another's development. Leading researchers review what is known about the dynamics of peer interactions and relationships from infancy through adolescence. Topics include methods of assessing friendship and peer networks; early romantic relationships; individual differences and contextual factors in children's social and emotional competencies and behaviors; group dynamics; and the impact of peer relations on achievement, social adaptation, and mental health. Salient issues in intervention and prevention are also addressed.