The research on children's involvement in social groups has seen a sharp increase from last decade. Much has been learnt about when children begin to display an interest in social groups, on what basis they join groups, how groups impact on the intra- and inter-group behaviors of members, and how children react to social group exclusion. This chapter outlines a social identity development theory (SIDT) that has sought to provide an account of children's involvement in social groups. According to SIDT, first and foremost for most children is the need to be accepted by a group and to feel that they belong to their group. Second, although social identity theory (SIT) provided a social motivational framework that, in principle, can be applied to the case of children, it focused on explaining prejudice and discrimination in adults. Children's play and interaction experiences with other children expand their growing knowledge about dyadic relationships, friendships, and interacting with groups of children.