International organizations' ability to respond promptly to crises is essential for their effectiveness and legitimacy. For the UN, which sends peacekeeping missions to some of the world's most difficult conflicts, responsiveness can save lives and protect peace. Very often, however, the UN fails to deploy peacekeepers rapidly. Lacking a standing army, the UN relies on its member states to provide troops for peacekeeping operations. In the first systematic study of the determinants of deployment speed in UN peacekeeping, we theorize that this speed hinges on the incentives, capabilities, and constraints of the troop-contributing countries. Using duration modeling, we analyze novel data on the deployment speed in 28 peacekeeping operations between 1991 and 2015. Our data reveal three principal findings: All else equal, countries that depend on peacekeeping reimbursements by the UN, are exposed to negative externalities from a particular conflict, or lack parliamentary constraints on sending troops abroad deploy more swiftly than others. By underlining how member state characteristics affect aggregate outcomes, these findings have important implications for research on the effectiveness of UN peacekeeping, troop contribution dynamics, and rapid deployment initiatives.