Under the Single Supervisory Mechanism of the European Banking Union, the ECB is responsible for the effective supervision of euro area credit institutions. To this end, the SSM Regulation entrusts the ECB with the task of applying Union law as well as national legislation transposing Union law. The application of national legislation by an EU institution is a complete novelty in European law and raises various legal questions. Which are the limits the ECB will have to respect when acting on the basis of national law? Can the ECB impose its own interpretation of national legislation? How should the judicial review of ECB decisions applying national legislation be organised? Scholars have raised several questions on the application of national law by the ECB, but no concrete answers have been developed thus far. Therefore, the ambition of this research project is to analyse the legal framework for the application of national legislation by the ECB, to identify legal issues and potential shortcomings, and to formulate normative proposals for further improvement.