The issue of constitutional authority, and more particularly the plurality of claims to legal and constitutional authority, has been a dominant theme of European Union legal scholarship in recent years. The resonance of the topic is evident in many of the major EU developments of the past decade: the momentous eastwards enlargement; the gambit of the unratified Constitutional Treaty; the growing number of national constitutional court challenges to EU authority claims; the likely EU accession to the European Convention on Human Rights; and finally the rulings of the European Court of Justice on the relationship of EU law to the international legal order.

When we were approached by John Haslam, editor at Cambridge University Press, with the suggestion that we put together a book of essays on the constitutional law of the EU, we embraced the opportunity he offered to invite a small number of the leading scholars in the field to write an in-depth essay on this compelling theme. The book is our second collaborative project, coming ten years after the publication of our first co-edited volume on the European Court of Justice.