In the process of European integration, national constitutional law remains crucial, in several respects. The EU Constitution, in its broadest sense, builds on the common constitutional traditions and principles of the Member States, and EU law refers back to national constitutional law in many ways. National constitutional law contributes to constitutionalising the EU, to give it legitimacy, and to ensure that the EU and the Member States as its agents continue to comply with fundamental constitutional requirements, such as the rule of law, democracy and accountability, and the protection of fundamental rights. Also, from the national perspective, national constitutional law remains essential, since it provides the foundation of membership of the EU, it is decisive for the procedures to be followed when powers are transferred at times of accession and Treaty amendment, and it serves to provide the connection between the national and the EU legal orders. Participation in the EU requires adaptation of the national constitutional arrangements, and national constitutions should not be neglected in the process of the constitutionalisation of Europe. This paper looks into the national constitutional experience of the EU-15 with the EU and asks whether lessons can be learned for candidate countries.