Many problems confronting today’s societies are transnational in character, leading states to increasingly rely on international organizations (IOs) for policy solutions. Yet the performance of IOs varies extensively. As an initial step toward explaining variation in IO performance, this article discusses how performance may best be conceptualized for empirical analysis. We advance an output-based account of IO performance, privileging the results IOs produce in terms of policy output. This conceptualization is distinct from the main alternative perspectives on IO performance, emphasizing either behavioral change by targets (policy outcome) or problem-solving effectiveness (policy impact). In addition, we introduce a typology of output that captures five generic features of IO policy of theoretical importance: policy volume, orientation, type, instrument, and target. We conclude by highlighting the implications of an output-based account to IO performance for the study of institutional design and effectiveness in global governance.