When, where, and how are the European Union’s (EU’s) supranational institutions able to advance their own political agenda? 1 This question is one of the central bones of contention in the study of European governance. Historically, theories of regional integration have offered competing answers, depicting the EU’s supranational institutions as either “engines of integration,” independently driving European cooperation, or “obedient servants,” effectively controlled by national governments. In recent years, scholars dissatisfied with these rival accounts have turned to institutional theory for the means to explain variation in supranational influence over time, institutions, and issue-areas.