Modern realism claims that the fear that others will enjoy relatively greater benefits frequently impedes international cooperation. Recent articles in thisReviewby Duncan Snidal and Robert Powell modeled conditions under which the impact of relative gains varied. Joseph Grieco criticizes Snidal's model as based on assumptions that allow him to avoid, rather than confront, the realist arguments. He also argues that Powell's model, while constructive, ignores important additional sources of sensitivity to relative gains. In response, Powell discusses the value of alternative assumptions about preferences and constraints in international relations. Snidal defends his analysis and presents an additional proof to support the independence of his central result—the diminishing impact of relative gains with increasing numbers of states—from assumptions of concern to Grieco. Both responders emphasize their work as contributing to a contextually rich theory of international politics that builds on elements of both realism and neo-liberalism.