Unmanned Aerial Vehicles (UAVs), more popularly known as drones, have become emblematic of twenty-first century military technologies but scholars have yet to convincingly explain the drivers of UAV proliferation. Using the first systematic data set of UAV proliferation, this research note examines the spread of UAVs in the context of scholarly debates about interests versus capacity in explaining policy adoption. The results yield important insights for both IR scholarship and the policy-making community. While countries that experience security threatsincluding territorial disputes and terrorismare more likely to seek UAVs, drone proliferation is not simply a function of the threat environment. We find evidence that democracies and autocracies are more likely than mixed regimes to develop armed UAV programs, and suggest that autocracies and democracies have their own unique incentives to acquire this technology. Moreover, supply-side factors play a role in the UAV proliferation process: a state's technological capacity is a strong predictor of whether it will obtain the most sophisticated UAVs. The theories and evidence we present challenge emerging views about UAV proliferation and shed useful light on how and why drones spread.