Sibling relationships are among the most enduring connections that individuals experience over their lifetimes. Given that approximately 80% of Western children have brothers and sisters, it is clearly a signifi cant relationship for young children. Yet, in the large theoretical and empirical literature on family relationships, siblings are frequently overlooked when explaining family dynamics. However, as we argue in this chapter, siblings play a critical role in family dynamics and are important developmental infl uences on one another. Earlier work (1950 -1970s) on sibling relations typically focused on structural variables (e.g., gender, age, and birth order), whereas more recent work has investigated the role of process in explaining developmental infl uences. This shift is evident in the kinds of questions posed, for example investigating the roles of different types of sibling interactions in the development of social understanding. This chapter focuses on sibling relations in early and middle childhood, but it is apparent that children are acutely aware of their younger sibling ' s presence from the time of their arrival in the family, and early patterns of interaction infl uence later development (see Dunn