This paper sets out the research agenda of the Kolleg-Forschergruppe “The Transformative Power of Europe. The European Union and the Diffusion of Ideas”. The diffusion of ideas has become a central research theme in political science, sociology, law, history, and economics. In this context, the KollegForschergruppe focuses on the theoretical and methodological challenges of identifying scope conditions for and interaction effects between the various causal mechanisms by which ideas are spread across time and space. We concentrate on the European Union (EU) as an almost ideal laboratory for investigating processes and outcomes of diffusion. First, European integration itself can be described as an effort to promote the diffusion of ideas across Europe and beyond. Second, European societies and polities emulate each other through mimetic processes. Third, Europe and the EU also serve as active promoters of diffusion processes toward the outside world. Last not least, European integration is embedded in and responds to larger global diffusion processes. The Kolleg-Forschergruppe will explore the diffusion of ideas in three thematic areas: “identity and the public sphere,” “compliance, conditionality, and beyond,” and “comparative regionalism and Europe’s external relations”.