This chapter explores the multifaceted nature of the application of national law by the ECB and the legal ramifications derived therefrom. The study aims, in particular, to briefly illustrate the wide-ranging set of issues—both actual and potential—arising from the complexity of Article 4(3) SSMR, involving not only the sphere of banking supervision, or EU financial law more broadly, but the entire EU judicial system. The effects of such a manifold relationship indeed touch upon the very structures of the EU’s institutional architecture. While the vast array of issues identified are far from having a concrete or definitive solution in the current state of affairs, the progressive evolution of legislation, judicial dialogue and case law, accompanying the exercise of regulatory and supervisory powers by the ECB—and, eventually, other EU agencies as well—, shall help clarifying at least part of this fascinating legal conundrum.