The chapter analyses how the ECB was granted responsibility over prudential supervision as part of the EU banking union. This changed the EU perspective on financial services from internal market issue to a macroeconomic financial stability issue. The ECB now supervises directly the largest banking groups in the euro area and monitors the supervision of all other banks as well. The ECB’s role in the banking union raises new types of constitutional questions and requires a different analytical framework. A formal legal assessment, particularly on the legal basis for conferring specific tasks on the ECB under Article 127(6) TFEU, is complemented with a broader economic constitutional analysis. Here, the context in which the banking union was initiated is particularly important. The chapter also discusses the merits and caveats of combining banking supervision and monetary policy, where the conflict of interest is the main caveat particularly for an independent institution such as the ECB. It is also concluded that the supervision could return to the internal market setting and again be separated from monetary policy in the future.