The authors review the literature on sibling relationships in childhood and adolescence, starting by tracing themes from foundational research and theory and then focusing on empirical research during the past 2 decades. This literature documents siblings' centrality in family life, sources of variation in sibling relationship qualities, and the significance of siblings for child and adolescent development and adjustment. Sibling influences emerge not only in the context of siblings' frequent and often emotionally intense interactions but also by virtue of siblings' role in larger family system dynamics. Although siblings are building blocks of family structure and key players in family dynamics, their role has been relatively neglected by family scholars and by those who study close relationships. Incorporating study of siblings into family research provides novel insights into the operation of families as social and socializing systems.