This work is designed to deepen our understanding of relationships within and between groups by means of three key ideas. The first is that groups are a source of social identity - our sense of who we are and what we are worth derive from our group memberships. The second is the constant tension in group life between its task and socio-emotional aspects - getting on with the job while also getting on with people . The third is that group dynamics are frequently governed by social comparison processes, both among the members within the group and between the ingroup and various outgroups. The book introduces readers to the major theoretical and empirical developments in the field of group dynamics. It draws together the very considerable research conducted into group behaviour by other psychologists and by the author himself. There are supporting illustrations from contemporary life, focusing on such important issues as crowd behaviour, social conformity, group productivity, ethnic prejudice and conflict, and co-operation between groups.