The introduction by the Amsterdam Treaty of the flexibility clauses, authorising a majority of Member States to cooperate more closely in areas covered by the Treaties, has been received with mixed feelings. Flexibility is by no means a new phenomenon in the EU's development. It has been on the Community's agenda already since the 1970s. The Single European Act introduced several provisions allowing for flexible approaches to the single market, whilst the Maastricht Treaty launched a differentiated approach to the EMU and social policy. In addition to these forms of differentiation in primary Community law, for many years there has also been a number of quite important, though less visible, forms of differentiation in secondary Community law. This book aims to link both levels of differentiation and seeks to unveil the many faces of differentiation in EU law. It analyses whether, and to which extent, there is a shift in European integration from a system of unity and uniformity to one of flexibility and differentiation. A first series of contributions to the book analyse a number of exemplary policy fields (EMU, social policy, environment, free movement of persons, justice and home affairs) in order to identify their degree of differentiation. A second set of contributions examine various 'horizontal' institutional matters of cross-sectoral importance. These two main parts are framed by introductory articles on the development of flexibility and by contributions drawing on the constitutional limits to differentiation. The contributions are made by Dominik Hanf, José M. de Areilza, Jean-Victor Louis, Sean Van Raepenbusch, Ludwig Kramer, Georgia Papagianni, Grainne de Burca, Ellen Vos, Linda Senden, Sacha Prechal, Wouter Devroe, Deirdre Curtin, Bruno De Witte, Eddy De Smijter en Jan Wouters.