Based on a theory of differentiated integration, this book investigates the dynamics of flexible European integration across EU policies and member states. By doing so, it provides a theory-based, comprehensive, and an empirical account of European integration from the perspective of legal differentiation.

Alexander Stubb, a participant in the 1996-97 and 2000 Intergovernmental Conferences analyzes the evolution of flexible integration from the early 1970s to the present day and beyond. He focuses in the process of negotiations which led to the institutionalisation of flexibility in the Amsterdam and Nice Treaties. This book provides a valuable insider's view on historical decisionmaking in the European Union.

Flexible Integration is a model of reform designed to overcome the current stalemate between federalists and anti-federalists. It introduces more flexibility to accommodate the heterogeneous interests in Europe without risking the gains achieved through past integration. Flexible integration combines firm commitment by all members to a supranational common baseincluding a well-defined set of competences related to the Single Market - with optional integration in other areas through open partnerships.

The entry into force of the Lisbon Treaty poses anew the question of whether, from now on, it will be more likely for the European Union to use this mechanism, or, conversely, the new institutional, jurisdictional and decision-making framework may act as a containment of favourable trends in the development of variable integration formulas. In reality, flexible or differentiated integration instruments, within or outside of the framework of the EU, alongside strictly intergovernmental cooperation between certain EU countries, have existed since the beginning of Community integration.

Flexibility is emerging as a key dynamic of European integration. This shift towards flexibility has major implications. The EU will have to cope with more complexity and less transparency. It also affects the way in which European integration is viewed since it makes a state-like outcome to the process far less likely. Alex Warleigh looks at why flexibility has become such an important feature of the EU. He examines its history, and puts forward a typology to explain the models by which it is understood. He goes on to explore the hazards of flexibility and to look at what it has to offer, arguing that it is best seen as a desirable part of the integration process rather than as a problem. Flexibility, he argues is an important mechanism for the realization of the EU's slogan "unity in diversity".

Author examines the new clause of 'flexibility' in the Treaty of Amsterdam and asks whether this clause will be sufficient to deal with the EU in the 21st century.