Whereas considerable research has focused on children's dyadic relationships (Bradford Brown & Dietz, 2011), comparatively little attention has been given to the intraand intergroup processes involved in children's experiences in groups, as well as issues relating to the structure and organization of children's groups (Cairns, Xie, & Leung, 1998). This lack of attention is surprising given the extent of children's involvement in groups, which commences prior to formal schooling and increases throughout the middle childhood years, reflecting the considerable importance of group membership to them (Rubin, Bukowski, & Parker, 2006). Indeed, the evidence suggests that if there is a possibility of being accepted by, and belonging to, a social group, children will seek to be included (Nesdale, 2007). Moreover, their social interactions occur increasingly within their social groups during the elementary school years (Rubin et al., 2006). Given the limited, albeit rapidly increasing, research attention given to children's social groups, it is perhaps unsurprising that theory development in this area has also been limited. However, the aim of the present chapter is to provide an outline of one theory, social identity development theory (SIDT; Nesdale, 2004, 2007),