De facto states, such as Somaliland (Somalia), are unrecognized separatist enclaves that display characteristics of statehood but lack an international legal status. To acquire domestic and external legitimacy, these actors engage in a wide range of governance practices: they set up military and police forces; executive, legislative, and judicial branches; hospitals; schools; banks; or social security networks. In spite of the obvious gains that can be accrued through the establishment of a complex governance architecture, de facto states exhibit great variation in the range of statelike institutions that they build: some, like Luhansk People’s Republic (Ukraine), put together a rudimentary governance apparatus, while others, like Transnistria (Moldova), manage to construct a complex system of rule. What explains the variation in governance practices across these separatist enclaves? Using original data on governance institutions across all de facto states (1945–2016), this study offers an empirical examination of the key factors that shape separatists’ incentives to supply governance. The findings reveal that de facto state separatists are less likely to provide governance when they have access to lootable mineral resources but are more likely to do so when they receive external military support, when peacekeepers are present, when they have access to relatively immobile assets, when they adopt a Marxist ideology, and when they control the territory for a long time. The findings help us better understand the conditions under which armed nonstate actors supplant sovereign states as de facto authorities and successfully institutionalize their rule.