This article examines the reform of the European Stability Mechanism (ESM) as an example of the hybrid character of EMU law. The article does so by analysing the functioning of the ESM and explaining the plans for its reform, including the troubled ratification process, within and outside the EU Treaty framework. The article then concludes stressing how the evolution of the ESM contributes to the consolidation of a hybrid EMU architecture, where the interaction between Treaty and non-Treaty norms raises a variety of legal and political challenges.