Does economic inequality breed political violence? For almost half a century, scholars have tried to test this assumption, finding little empirical support for a statistical relationship between the two variables. This article provides a critical review of this literature, starting out with the link between so-called vertical (inter-individual) inequality and conflict. I argue that the lack of empirical results can largely be attributed to the almost exclusive focus on individual-level differences in terms of income or land. Group identity is critical to recruitment and maintaining allegiance to a military organization. Hence, we should focus the attention on the relevant form of inequality—that between groups, or so-called horizontal inequalities. In contrast to the studies focusing on vertical inequality, an emerging quantitative literature on horizontal inequalities and conflict has found a positive link, which is more in line with the evidence from several case studies. However, measuring horizontal inequalities is a clear challenge, and there is a need for additional studies to qualify the initial findings. I conclude by suggesting some avenues for future research.