This book argues that political and economic inequalities following group lines generate grievances that in turn can motivate civil war. Larks-Erik Cederman, Kristian Skrede Gleditsch, and Halvard Buhaug offer a theoretical approach that highlights ethnonationalism and how the relationship between group identities and inequalities are fundamental for successful mobilization to resort to violence. Although previous research highlighted grievances as a key motivation for political violence, contemporary research on civil war has largely dismissed grievances as irrelevant, emphasizing instead the role of opportunities. This book shows that the alleged non-results for grievances in previous research stemmed primarily from atheoretical measures, typically based on individual data. The authors develop new indicators of political and economic exclusion at the group level, and show that these exert strong effects on the risk of civil war. They provide new analyses of the effects of transnational ethnic links and the duration of civil wars, and extended case discussions illustrating causal mechanisms.