Why do rebels vary their tactics? Some insurgents employ terrorism and hit-and run attacks; others wage conventional wars against state rivals. I argue rebels’ tactical choices reflect three constraints: economic opportunities of non-combatants, state strength, and rebel capacity. I test the argument with microdata on rebel violence in Colombia and exploit plausibly random shocks to local income. I find evidence that local economic shocks substantially affect rebel tactics. Specifically, when government forces benefit from local windfalls and economic opportunities for civilians improve, insurgents favor irregular tactics. Alternatively, when rebels are strengthened, they favor conventional tactics. These results are robust to accounting for numerous potential sources of bias, including atmospheric dispersion of illicit crop herbicides, violence spillovers from drug trafficking, and foreign military aid shocks. The main findings challenge prominent theories of insurgency, and are relevant to the comparative study of political violence, with important implications for scholars and policy makers.