This collection of essays aims to address the changing constitutional framework of the European Union, and some of the changing patterns of governance within this complex polity. The primary aim of the book is to examine the apparent and gradual shift in the paradigm of European governance from one emphasizing the importance of uniformity and harmonization to one which embraces a substantial degree of flexibility and differentiation. The chapters range from broad, theoretical reflection on the constitutional implications of differentiation and flexibility for the European polity, to more focused case studies which examine various forms of closer co-operation, variable geometry and flexibility existing in specific policy areas. Some of the contributions interrogate the extent to which there has actually been any significant change of paradigm, and others explore the many different meanings and instances of flexibility which have emerged. Overall, in presenting a variety of perspectives and methods of addressing these important and topical questions, the collection brings into focus both the problems and the potential ways forward for Europe which these constitutional developments suggest.