First published in 1982. Since the emergence of developmental psychology early this century, theorists and researchers have emphasized the family’s role in shaping the child’s emergent social style, personality, and cognitive competence. In so doing, however, psychologists have implicitly adopted a fairly idiosyncratic definition of the family— one that focuses almost exclusively on parents and mostly on mothers. The realization that most families contain two parents and at least two children has occurred slowly, and has brought with it recognition that children develop in the context of a diverse network of social relationships within which each person may affect every other both directly (through their interactions) and indirectly (i.e., through A ’s effect on B, who in turn influences C). The family is such a social network, itself embedded in a broader network of relations with neighbors, relatives, and social institutions. Within the family, relationships among siblings have received little attention until fairly recently. In this volume, the goal is to review the existing empirical and theoretical literature concerning the nature and importance of sibling relationships.