Siblings have considerable influence on one another's development throughout childhood, yet most human development research has neglected sibling socialization. Through this volume, we aim to enhance our understanding of how siblings play formative roles in one another's social and emotional development. We examine the mechanisms by which children are influenced by their brothers and sisters, clarify the ways in which these mechanisms of socialization are similar to and different from children's socialization experiences with parents, and consider the conditions under which sibling socialization results in positive versus negative consequences for individual development.