This paper describes and contrasts three contemporary social psychological theories that focus on why people join and identify with groups: the sociometer model, terror management theory, and uncertainty–identity theory. The sociometer model argues that people have a need to be socially included, and that self-esteem is a meter of successful inclusion and group belonging. Terror management theory argues that people are motivated to reduce fear of the inevitability of their own death, and that the consensual belief–confirmation provided by groups drives people to belong. Uncertainty–identity theory argues that people have a basic need to reduce uncertainty about themselves, their attributes, and their place in the world, and that cognitive processes associated with group identification reduce such uncertainty. We critically contrast these three accounts to conclude that all three motivational processes may play a role, but that self-uncertainty may have the benefit of wide generality to all groups and group contexts and of detailed specification of cognitive processes.