Book excerpt: A definitive reference on intra- and inter-group processes across a range of age and cultural contexts Children from infancy develop attachments to significant others in their immediate social environment, and over time become aware of other groups (e.g. gender, ethnicity, age, classroom, sports) that they do or do not belong to and why. Recent research shows that children’s attitudes, beliefs and behaviours are significantly influenced by these memberships and that the influence increases through childhood. This Handbook delivers the first comprehensive, international reference on this critical topic.