Why are ethnic movements more likely to turn violent in some multiethnic countries than in others? Focusing on the long-term legacies of European colonialism, this book presents two ideal-typical logics of ethnic group mobilization—one of violent competition and another of nonviolent emancipatory opposition. The book’s theory first explains why ethnic grievances are translated into either violent or nonviolent forms of conflict as a function of distinct ethnic cleavage types, resulting from different colonial experiences. Violent intergroup conflict is least likely where settler colonialism resulted in persistent stratification, with ethnic groups organized as ethnoclasses. Such stratified societies are characterized by an equilibrium of inequality, in which historically marginalized groups lack both the organizational strength and the opportunities for armed rebellion. In contrast, where colonialism and decolonization divided ethnic groups into segmented, unranked subsocieties that feature distinct socioeconomic and cultural institutions, ethnic mobilization is more likely to trigger violent conflict. Second, the theory links this structural explanation to the political actors at the heart of ethnic movements—in particular, ethnic organizations. It elucidates how these organizations fuel the risk of civil conflict in segmented unranked societies, but peacefully promote the empowerment of historically marginalized groups in stratified societies. The book draws on an innovative mixed-methods design that combines large-n statistical analyses—using new data on the linguistic and religious segmentation of ethnic groups, as well as on ethnic organizations—with case studies based on original field research in four different countries in sub-Saharan Africa and Latin America.