The study of siblings has become increasingly central to developmental science. Sibling relationships have unique effects on development, and sibling designs allow researchers to isolate causal mechanisms in development. This volume emphasizes causal mechanisms in the social domain. We review the preceding chapters in relation to six topics: a multilevel modeling approach to the ecology of sibling relationships, unique contributions of sibling relationships to development, sibling similarity and dissimilarity, developmental stages, culture and class, and intervention studies with siblings. We conclude with insights regarding directions for future research.