The association between horizontal inequalities (HIs) and civil war is well established. Yet, studies of HIs and civil war rarely specify why HIs should lead to conflict that is violent, organized, and large-scale, such as civil war. Collective mobilization outside conventional political channels could take many other forms, such as demonstrations, strikes, riots or communal conflicts. Recently, researchers have begun to investigate whether HIs increase the risk of nonviolent, less organized, and smaller-scale conflict. This article provides the first systematic review of the emerging quantitative literature on the impact of HIs on both violent and nonviolent conflict mobilization. We ask: Do HIs increase all mobilization outside conventional political channels, or do they shape what form of conflict breaks out? We supplement previous reviews of the literature on HIs and political violence with a systematic update for scholarly articles published in 2017–2022. This yielded 22 new articles. Furthermore, we identify 20 articles on HIs and nonviolent conflict published in 1996–2022. The review reveals that while there is robust evidence on the relationship between HIs and violent mobilization, the emerging evidence on nonviolent conflict is inconsistent. We conclude by discussing potential reasons for this inconsistency.