Studies of international institutions, organizations, and regimes have consistently appeared in the pages of International Organization. We review the theoretical and empirical work on international institutions and identify promising directions for the institutionalist research program. Early studies of international institutions were rich with empirical insights and often influenced by theoretical developments in other fields of political science, but lacking an overarching analytical framework they failed to produce a coherent body of scholarship. Current efforts to reinvigorate the study of international institutions draw on a new body of theory about domestic institutions. We argue that the assumptions of this new approach to institutions are more appropriate to international studies than those of earlier attempts to transfer theories across levels of analysis. We suggest that the most productive questions for future research will focus on specifying alternative mechanisms by which institutions can influence outcomes and identify particular sets of questions within this agenda that are especially promising.