The main instrument of constitutional change in the European Union, during the last twenty years, was the revision treaty, that is, an international agreement between the member states of the European Union making amendments to the so-called ‘founding treaties’. The recent adoption of a European Constitution was seen by all the leading actors, and without any controversy, as involving again a revision of the current Treaties in accordance with the procedure of Article 48 EU Treaty, rather than the creation of a wholly new legal edifice. This Constitution thus fits, despite its many novelties, in the long chain of European revision treaties. All the revision treaties were important steps in the development of the European integration process. They were also, from a legal perspective, cases of amendment of multilateral treaties subject to the legal regime set out in Articles 39 to 41 of the Vienna Convention on the Law of Treaties.

European treaty revision was a success story in what could be called its golden age (1985–2000), when amendments were successfully made and implemented in accordance with the traditional method of intergovernmental conferences followed by national ratifications. However, increasing criticism of this traditional method led, not to the rejection of the international law framework altogether, but to claims for reform of the particular expression of that international law regime in the EU context. Those misgivings related to the exceedingly diplomatic (and therefore opaque) character of treaty negotiations, and to the excessive rigidity caused by the ‘double lock’ of overall consensus by all governments at the intergovernmental conference, followed by universal ratification in all the member states. The article examines the recent debates on these points, as well as the remedies which the new European Constitution adopted or failed to adopt.