The paper reviews key aspects of the new constitutional framework for the European Union, once the Treaty of Amsterdam has been ratified, in the light of the core challenges of managing flexible integration in an enlarged Union and securing adequate legitimacy for the integration project. Reviewing briefly the general debates on flexibility, and its relationship to different constitutional and political futures for the Union which are suggested by those involved in the debates, the paper examines the principal provisions governing what is termed `closer cooperation' within the new Union treaties. The emphasis is placed on the framework provisions of the TEU, and those in the First Pillar. It is noticeable that the Treaty takes a `non-ideological' approach to flexibility, eschewing direct support for those who interpret flexibility as meaning more or less integration in the future. It provides a framework for future cooperation which is likely to be too restrictive to be workable, except in very limited circumstances. However, particular instances of flexibility are provided in the Treaty, in the form of the opt-outs from the new free movement title and the communitarisation of Schengen for the United Kingdom, Ireland and Denmark, and some might even describe these as `pick-and-choose'. The paper concludes by reviewing the flexibility debate against the background of the ongoing legitimacy challenge for the Union, arguing that, as currently conceived, flexibility is more to do with balancing political interests than with securing or enhancing legitimacy.