Why do states contribute to alliances? Is relative size the principal factor influencing the size of contributions, as many studies suggest, or are perceptions of threat, dependencies on other alliance members, and domestic institutions and policies equally important? These questions hold unusual interest in the wake of the cold war. The end of bipolarity promises more ad hoc coalitions, which will widen opportunities for research on alliance burden-sharing beyond the North Atlantic Treaty Organization (NATO). At the same time, because the political fault lines of the cold war have disappeared, there are few accepted political criteria for sharing those security burdens that are perceived collectively.