Scholars and policymakers alike have recently begun to tout the ability of international organizations (IOs) to encourage and secure democracy throughout the world. Despite this stance, little theoretical attention or empirical investigation has attempted to ascertain why or whether this relationship truly exists. One challenge to answering this puzzle is that extant theories of international institutions do not generally delineate clear hypotheses about how IOs influence domestic politics. In this article, I address this paucity of both theory and empirical evidence. I delineate three causal mechanisms that link IOs to domestic actors' calculations about political liberalization and test the argument. I find that membership in regional IOs is correlated with transitions to democracy during the period from 1950 to 1992.