Book excerpt: How and under what conditions does the European Union (EU) shape processes of institution building in other regional organizations? This book develops and tests a theory of interorganizational diffusion in international relations that explains how successful pioneer organizations shape institutional choices in other organizations by affecting the institutional preferences and bargaining strategies of national governments. The author argues that Europe's foremost regional organization systematically affects institution building abroad, but that such influence varies across different types of organizations. Mixing quantitative and qualitative methods, it shows how the EU institutionally strengthens regional organizations through active engagement and by building its own institutions at home.