This paper deals with two litmus tests for theories of European integration. The first part asks, how and to what extent various approaches can explain the contemporary crises of European integration. It thereby tackles the question whether European integration theories might have biased EU scholars towards ignoring evidence for (dis-)integration. While being more optimistic about the state of the Union than many EU scholars are, the paper argues for a more differentiated conceptualization of integration as a continuous variable that takes disintegration rather than stagnation or no integration as the opposite value of integration. The second part of the paper asks to what extent European integration theories are able to shed light on experiences with regionalism across the globe. It argues that they do provide plausible accounts for the emergence of regionalism around the world. Comparing regions points to important scope conditions under which European integration theories operate. When it comes to outcomes, however, they need to be complemented by explanations emphasizing diffusion to explain why and when states are more inclined to pool and delegate sovereignty in some regions than in others.