The overall themes of this book are past and present changes in European co-operation. Two such changes were at the top of the agenda in the summer of 2004: the Eastern enlargement and the constitutional treaty. A way to interpret these changes is to look at them as manifestations of the two core concepts, ''diversity'' and ''unity''. The combination of the concept of unity with the apparently contrasting notion of diversity - the two extremes of a scale seems to amount to a contradiction. But that is exactly the nature of European co-operation. How to overcome this contradiction and make it fruitful and productive is the challenge faced by the nations of Europe in the 21st Century. The different chapters address different aspects of this challenge: from the problems faced by new member states of Central Europe, via the long and tortuous road towards a common European constitution and popular Euro-scepticism, to the role of Europe in the world as mirrored by its relations with the United States of America. Enlargement and constitutional treaty are the two keywords of the European Union in the 21st century. And they incarnate the two notions of ''diversity'' and ''unity''. They both constitute major changes in the architecture of the European Union. The first change - enlargement - is a major step towards the overall goal of uniting all of Europe, of making the map of the Union congruent with that of geographical Europe. What began as a mere economic co-operation between six West European countries now almost covers the whole continent from the Atlantic to the Russian border. The chapters are the written versions of six of the lectures given at an international summer university under the theme "Europe and the European Union - a Scenario for the Future", held at Copenhagen Business School. The seminar was organised as a co-operation between the Centre for the Study of Europe at CBS and the European Studies Program at Malmö University under the auspices of the Oeresund University.