This chapter 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 of 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 authors of this chapter argue 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 chapter examines 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 clarify why and when states are more inclined to pool and delegate sovereignty in some regions than in others.