Although children's peer relationships have been the subject of research attention since at least the 1920s, it is only since the 1970s that systematic theories have been proposed to integrate the rapidly growing body of empirical knowledge. Children are involved in a variety of relationships with parents, other adults, siblings, peers and so on. Two simple characteristics which distinguish children's peer relationships from their other relationships are their levels of equality and power. Relationships with peers are relationships broadly among equals. Both parties benefit from and contribute to the relationship in roughly equal measure and there is a broad balance of power. In contrast, many of a child's relationships are more hierarchically structured and are decidedly unequal. Older children and adults, especially parents, have considerably greater knowledge, resources and power and so are much more able to determine the course of a relationship and the relative outcomes for themselves and the child. The focus of this book is on children's relationships with their peers and, more specifically, children's friendships - their well-established reciprocal relationships with peers. However, these relationships do exist within a broader social context which includes a range of previous and current other social and personal relationships and so it is also important to bear these potential influences in mind if we are to fully understand the nature and significance of children's friendships.