The European Union is a new subject for theories of legitimacy, posing fundamental questions to the established concepts and principles of democratic theory. General compliance and popular acceptance and respect for European law is at stake. The volume addresses the main challenges of the European Union to democratic theory. The legitimacy of such transnational institutions born by political integration has so far received some but scant attention. The mere existence of the Union proves that the sovereign state cannot remain the sole focus of normative reflection. Indeed, the very conception of sovereignty is at stake. The present volume combines political science and normative political theory to offer concepts, arguments and criteria that further these debates, addressing problems of principle.