One of the most striking changes contained in the Maastricht Treaty was the establishment of a de jure `citizenship of the Union'. For the first time since the Roman Empire, peoples of Eastern, Western, Northern and Southern Europe share a common legal citizenship status. The significance of this development is potentially profound, yet it is one of the least discussed aspects of the Maastricht Treaty. In this book Stephen Hall examines the legal implications of establishing a European citizenship. He shows that Community law has never given unqualified effect to the Member States' dispositions of their nationalities, and that the Member States have had their sovereign power to confer and withdraw their nationalities qualified by the Maastricht Treaty. The book goes on to discuss the implications of Union citizenship on Community migration rights, demonstrating that the new non-economic migration rights for Union citizens are directly effective but that they are subject to a range of important limitations and conditions. Among these conditions is a residual constitutional power, contingently retained by the Member States to derogate from Community rights on the grounds of national or public security.