This chapter suggests a social psychological framework for understanding populism’s catch-all political appeal and invites the reader to appreciate the significance of identity construction in populist mobilization. By politicizing an existing but otherwise dormant social identity, populist entrepreneurs capitalize on the normative weight of the principle of popular sovereignty to produce a resonant and all-encompassing collective identity, which they proceed to rigorously police against competing social identities that undermine their aggregative objectives. Populism is fueled by the powerful social psychological mechanisms of intergroup polarization and ingroup favoritism inherent in a particularly acute “us-them” (“people”-vs-“elites”) antithesis, allowing its proponents to upset long-established party systems and to potentially decompose partisan identity structures built along ideological, class-based, ethnic, religious, or other types of social cleavages.