While I write this chapter, millions of people in the Darfur province of Sudan have been terrorized off their land; the entire population of Iraq has little idea what the future of their country will be; survivors of hurricane Katrina are dispersed across the United States; people in Britain are anxious about immigration and are toying with the idea of supporting the British National Party; people in a small town in Tasmania wait to hear if members of their community have been found alive in a mine collapse; air travelers the world over have no idea what new security arrangements await them when they get to the airport; and we all wonder about the consequences of further escalation in the price of oil and of the standoff over Iran's uranium enrichment program. The world is an uncertain place, it always has been, and these uncertainties can make it very difficult to predict or plan our lives and to feel sure about the type of people we are.In this chapter, I describe how feelings of uncertainty, particularly about or related to self, motivate people to identify with social groups and to choose new groups with, or configure existing groups to have, certain properties that best reduce, control, or protect from feelings of uncertainty. I consider this uncertainty–identity theory to be a development of the motivational component of social identity theory. It addresses why, when, and how strongly people identify with groups, and why groups may have particular generic properties in certain contexts. Of particular relevance to contemporary postmodern society, uncertainty reduction theory provides an account of zealotry and the cult of the “true believer” in the thrall of ideology and powerful leadership—an account of conditions that may spawn extremism, a silo mentality, and a loss of moral or ethical perspective.In this chapter, I describe uncertainty–identity theory and some conceptual elaborations and applications, review direct and indirect empirical tests, and locate the theory in the context of related ideas and theories in social psychology. I start with a historical sketch of why, when, and how uncertainty–identity theory was developed, then go on to discuss uncertainty reduction as a motivation for human behavior. I then detail the process by which group identification reduces uncertainty and describe a program of studies showing that people who feel uncertain are more likely to identify and identify more strongly with groups. High‐entitativity groups are best equipped to reduce uncertainty through identification—entitativity moderates the uncertainty–identification relation. I discuss this idea and describe research that supports it, and then extend the analysis to deal with extremism and totalistic groups—describing how extreme uncertainty may encourage strong identification (zealotry, fanaticism, being a true believer) with groups that are structured in a totalistic fashion. Again I describe some research supporting this idea.The next section deals with extensions, applications, and implications of uncertainty–identity theory. I discuss the relation between depersonalization and self‐projection processes in uncertainty‐motivated group identification, and then, in a subsection entitled central members, marginal members, leaders, and deviants, I focus on the role of group prototypicality in uncertainty reduction processes. The role of trust, the relation between uncertainty, identity, and ideology, and the role of uncertainty in social mobilization are also discussed. The final section, before concluding comments, discusses other theories, approaches, and topics that deal with constructs related to those discussed by uncertainty–identity theory. Specifically, I discuss uncertainty as a state versus a trait, with a focus on the constructs of need for cognitive closure and uncertainty orientation; the role played by culture in uncertainty; and the relevance of terror management, compensatory conviction, self‐verification, and system justification.