Over the last decades, the concept of identity has become increasingly central in the social psychology of protest. Collective identity, politicized collective identity, dual identity, and multiple identities are concepts that help to understand and describe the social psychological dynamics of protest. In this article, I theorize about identity processes in the context of protest participation: how group identification establishes the link between social identity and collective identity, how multiple identities and dual identities influence protest participation, and how collective identity politicizes and radicalizes. I will illustrate my argument with results from research into collective action participation among farmers in the Netherlands and Spain, Turkish, and Moroccan immigrants in the Netherlands and New York, South African citizens, and participants in street demonstrations conducted by my research group at VU-University. © 2013 International Society of Political Psychology.