How does the Parliament of the United Kingdom hold ministers to account for their European policies? How does the EU principle of subsidiarity affect the relation between Westminster and the Scottish Parliament? How does Belgium accomodate its powerful regional assemblies into the European role of its federal parliament? What is the role of the Dutch parliament before and after the popular rejection of the Constitutional Treaty in the Netherlands? How does the parliament of Croatia view all these developments as it prepares for accession to the EU? And how should academics in general conceptualise the role of national parliaments in Europe's complex constitutional system, before and after the Constitutional Treaty? As national and regional parliaments get explicitly involved in the European Union, this volume addresses some key notions that are highly relevant to the ongoing debate: parliamentarism and constitutionalism, legitimacy and transparency, subsidiarity and devolution. This volume will hold great value for academics and policy-makers in both the European Union and the Member States.