Rebel groups that confront the government frequently become engaged in fierce and violent struggles with other groups. Why does a rebel group who is already fighting with the government become engaged in yet another struggle, thereby sacrificing scarce resources in the fight against other rebel groups? This article addresses this puzzle by providing the first global study on the determinants of interrebel violence. The authors argue that this violence should be understood as a means to secure material resources and political leverage that can help the group prevail in the conflict with the government. The quantitative analysis builds on new data on armed conflict between nonstate actors, 1989‰ÛÒ2007. The results show that interrebel conflict is more likely when the rebel group fights in an area with drug cultivation, when the group is in control of territory beyond government reach, when the group is either militarily strong or weak in relation to other rebels, and where state authority is weak. Why do some civil conflicts see fierce and sustained fighting between groups on the nonstate side, whereas other conflicts do not? Armed conflicts between rebel organizations have been prevalent in, for example, the Myanmar, Afghan, Liberian, and Sudanese conflicts. These incidents are not idiosyncrasies.