In order to explain the emergence of violent conflicts, an increasing number of academic studies have focused on the role of inequalities between ‘culturally’ defined groups or ‘horizontal inequalities’. The concept and theory of horizontal inequalities is also gaining purchase in donor agencies and the broader international development community, particularly in the context of specific countries undergoing or recently emerged from violent conflicts in which such inequalities appear to have played an important role. In this article, we critically review the scope and evidence for the relationship between the presence of severe horizontal inequalities and the emergence of violent conflicts, and lay out areas demanding further research. We identify three main avenues for future research which could extend the horizontal inequality approach and deepen its analytical advances by focusing more attention away from the state. Two of these relate to the role of non-state centred dynamics in the evolution and interpretation of horizontal inequalities and their mobilisation capacity; the third relates to the broad sociological process through which horizontal inequalities and ethnic identity formation may, in fact, be iteratively or dialectically intertwined.