The ‘new institutionalism’ is a term that now appears with growing frequency in political science. However, there is considerable confusion about just what the ‘new institutionalism’ is, how it differs from other approaches, and what sort of promise or problems it displays. The object of this essay is to provide some preliminary answers to these questions by reviewing recent work in a burgeoning literature. Some of the ambiguities surrounding the new institutionalism can be dispelled if we recognize that it does not constitute a unified body of thought. Instead, at least three different analytical approaches, each of which calls itself a ‘new institutionalism’, have appeared over the past fifteen years. We label these three schools of thought: historical institutionalism, rational choice institutionalism, and sociological institutionalism.’ All of these approaches developed in reaction to the behavioural perspectives that were influential during the 1960s and 1970s and all seek to elucidate the role that institutions play in the determination of social and political outcomes. However, they paint quite different pictures of the political world. In the sections that follow, we provide a brief account of the genesis of each school and characterize what is distinctive about its approach to social and political problems. We then compare their analytical strengths and weaknesses, * An earlier version of this paper WLS presented at the 1994 Annual Meeting of the American Political Science Association and at a Conference on ‘What is Institutionalism Now? at the