Liberal Intergovernmentalism (LI) is the contemporary “baseline” social scientific and historiographic theory of regional integration—especially as regards the European Union. It rests on three basic assumptions, which in turn support a three-stage theoretical model of integration and the elaboration of numerous distinctive causal mechanisms. Considerable historical and social scientific evidence supports the LI view, but room also remains for scholars to extend and elaborate its framework in promising ways. Three prominent criticisms of LI exist. Some scholars of “administrative politics” charge that it applies only to treaty-amending decisions and other rare circumstances. “Historical institutionalists” charge that it overlooks endogenous feedback from previous decisions. “Post-functionalists” and “constructivists” revive discredited claims from the 1960s that functional theories neglect the central role of identity claims and ideology in explaining national interests. While each criticism contains some truth, LI possesses rich theoretical resources with which to address them fruitfully and musters compelling evidence to support its empirical claims. This confirms LI’s preeminent role in scholarly debates and suggests a soberly optimistic future for European and regional integration.