In no other policy field has European integration advanced more rapidly over the past decade nor has the challenge of reconciling uniform rules with national diversity arisen more sharply than in financial regulation generally and banking supervision in particular. The creation of the Single Supervisory Mechanism (SSM) for European Banking Union is widely recognized as one of the most significant integration steps since the Maastricht Treaty and the introduction of the euro. This paper begins by outlining two alternative approaches to integrating diversity within the EU: differentiated integration (DI), the adoption of policies and rules that apply only to some Member States, but not to others; and experimentalist governance (XG), a process of provisional goal setting and revision, based on recursive learning from comparative review of implementation in different local contexts. The paper then goes on to analyze the SSM from three distinct perspectives: first, as a centralized hierarchy, seeking to impose and enforce uniform rules, standards, and procedures across the Banking Union; second, as a polyarchic network, seeking to orchestrate intensive cooperation between the ECB and the NCAs; and finally, as an experimentalist organization, seeking to accommodate and learn from diversity by adapting common rules and procedures to the specificities of individual banks, and revising them regularly through peer review of implementation experience at multiple levels. Drawing on a wide range of documentation and in-depth interviews with EU and national officials, the paper reconstructs the evolution of the SSM’s organizational practices and decision-making processes, along with its institutional structures, from its inception in 2014 to the present. From this study of the SSM “in action”, the paper argues that while each of these three views highlights important characteristics which need to be incorporated into any comprehensive picture, the most encompassing perspective is that of the SSM as an experimentalist organization, which integrates and recasts key elements of the other two views into a more complete and dynamic analysis of its evolving architecture and practical operations. The paper concludes by drawing out some broader implications from the case of the SSM for the relationship between uniformity, differentiation, and experimentalism in EU regulation more generally
