On 1 May 2004, the European Union embarked on a historic and in many respects unprecedented expansion. It admitted ten new countries, predominantly from Central and Eastern Europe (CEE) which, for half a century, had been separated by the Iron Curtain. One of many aspects that make this enlargement special is that the acceding countries regained their sovereignty only a little more than a decade ago. In response to a painful past, the new constitutions of Central and Eastern Europe accord a prominent status to sovereignty and independence, and were notably closed to the transfer of powers to international organisations. In order to join the European Union (EU), these constitutions therefore needed to be ‘opened up’, and the countries engaged into a major process of constitutional revision to enable the transfer of a part of their sovereignty to a highly integrated supranational organisation. This proved to be a sensitive and controversial exercise, not least because unlike previous enlargements, these countries joined at a time when the EU has been engaged in a major constitutional reform involving on occasions federal undertones. This book explores the amendments against the background of comparative experience and theory of sovereignty, as well as in the context of political sensitivities, such as rising euroscepticism ahead of accession referendums. It also undertakes a broader inquiry into the role and rationale of the national constitutions in the process of European integration, as well as exploring the implications of the European Constitution.