International institutions have become an increasingly common phenomenon of international life. The proliferation of international organizations (IOs) (Shanks et al., 1996), the growth in treaty arrangements among states (Goldstein et al., 2000) and the deepening of regional integration efforts in Europe all represent formal expressions of the extent to which international politics has become more institutionalized.
The scholarship on international institutions has burgeoned in response. Moreover, in the past decade, theories devoted to understanding why institutions exist, how they have functioned and what effects they have on world politics have become increasingly refined and the methods employed in empirical work more sophisticated. The purpose of this chapter is to draw together this divergent literature, to offer observations on the development of its various theoretical strands and to examine progress on the empirical front. We predict that a broad range of theoretical traditions – realist, rational functionalist, constructivist – will exist alongside one another for many years to come, and offer some suggestions on research strategies that might contribute to a better empirical base from which to judge more abstract claims.