A motivational extension of social identity theory is proposed: the uncertainty reduction hypothesis. Building on social identity theory and self-categorization theory, a subjective uncertainty reduction model of motivation associated with social identity process and group and intergroup behavior is developed and described. Contextually generated subjective uncertainty about important, usually self-conceptually relevant, matters motivates uncertainty reduction. The processes of self-categorization and prototypical depersonalization responsible for social identification and group behaviors are well suited to subjective uncertainty reduction; they contextually assimilate self to a prescriptive prototype that guides and consensually validates perception, cognition, affect and behavior. Group membership, social category-based self-conceptualization, group behavior, and intergroup relations are motivated by uncertainty reduction. Contextual uncertainty can be reduced by group membership and group action. This model integrates self-enhancement and self-evaluative motives into a single motivational framework for social identity processes. Derivation and explanation of the model recruits literatures on social identity, self-categorization, uncertainty, social comparison processes, self-motives, self-esteem, uncertainty related motives. sociostructural motivations, intragroup processes, intergroup relations, extremism, prototypicality, entitativity. social influence, and social change. Some direct tests of the uncertainty reduction hypothesis are described.