In this book, you will find a presentation of the numerous contexts of flexible co-operation in the European Union.The Treaty of Amsterdam’s closer co-operation paragraphs had the merit of putting a long standing practice of diversity not only into a new straightjacket but also into a new perspective. However it also created new sources of confusion. The Treaty of Nice brought some modifications but no great clarification, leaving the latter to legal practice and, in its absence, to scholarship. This is the challenge picked up by the present book. Its specific approach is to find analogies for these novel EU techniques of flexibility in past of existing but experienced constitutional systems, either confederate or federal. The book combines legal analysis with historical detail. In this book, you will find a presentation of the numerous contexts of flexible co-operation. The main topics to come out of the debates were 1) the necessity of leadership or dominance in closer co-operation projects; 2) the usefulness of the flexibility provisions; 3) the control and transparency of enhanced co-operation forms; 4) the way they lead to unity or diversity. These issues come together in the epilogue by W.T. Eijsbouts, (European Law, University of Amsterdam) who, notwithstanding all complexity, detects a model of ‘balanced constitution’.