In his chapter Nicholas Haagensen seeks to articulate the processes of legal construction and elaboration in a novel area of EU financial and monetary law. Using qualitative methods, he explains how EU lawyers, first, built a novel legal structure – the European Stability Mechanism (ESM) – during the Eurozone crisis in the years 2010 to 2012, and second, how they defended their construction before the CJEU. While attempting to resolve the legal tensions arising between the EU legal order and the ESM legal framework, the CJEU backed the mutation of Europe’s Economic and Monetary Union (EMU) with its ruling in the Pringle judgement. The establishment of the ESM – an international financial institution – outside EMU, while still being intimately connected to the EU legal order, instigated a mutation of EMU through the introduction of international law modalities into its EU law foundations. Drawing on interviews with lawyers from the European Commission, the Council, the European Parliament and the ECB, as well as the legal observations of all the parties to the Pringle case, the chapter explains and maps the process of legal construction and elaboration of changes to the EMU legal structure.