This article by Gudrun Østby features in the World Social Science Report 2016. This contribution summarizes the literature on how inequality relates to political conflict. It reviews the most central theoretical arguments and empirical findings concerning this relationship. Most importantly, there is a growing consensus that whereas inequality between individuals (vertical inequality) does not affect the risk of conflict, systematic inequality between identity groups (horizontal inequality) indeed does. Further, the article discusses various approaches to measuring horizontal inequality. It ends by discussing some avenues for policy and future research.