Why is it that some social movements engaged in contentious politics experience radicalization whereas others do not? The Dynamics of Radicalization offers an innovative reply by investigating how and when social movement organizations switch from a nonviolent mode of contention to a violent one. Moving beyond existing explanations that posit aggressive motivations, grievances or violence-prone ideologies, this book demonstrates how these factors gain and lose salience in the context of relational dynamics among various parties and actors involved in episodes of contention. Drawing on a comparative historical analysis of al-Qaeda, the Red Brigades, the Cypriot EOKA, the authors develop a relational, mechanism-based theory that advances our understanding of political violence in several important ways by identifying turning points in the radicalization process, similar mechanisms at work across each case, and the factors that drive or impede radicalization. The Dynamics of Radicalization offers a counterpoint to mainstream works on political violence, which often presume that political violence and terrorism is rooted in qualities intrinsic to or developed by groups considered to be radical.