Irrespective of whether the Treaty establishing a Constitution for Europe or a Reform Treaty enters into force, the EU already has a constitution by all but name. But what does it rest on? Both the current treaties and the European Court of Justice often make reference to the national constitutions, either of one Member State individually, or as common constitutional principles and traditions. Yet, whilst the influence of EU law on national constitutional law is well documented, no thorough comprehensive comparative legal research has ever been done into these common constitutional principles. The very foundations of the European constitution have thus remained uncharted. Trans-national comparative constitutional law has been neglected in the scientific research on the European Constitution, while it should be an essential component of the analysis. This project aims to contribute to the scientific debate by going back to the fundamentals of national constitutional law. Its purpose is to analyse and structure common legal constitutional principles across EU Member States and identify constitutional diversity. To that end a team of PI and four post-docs will conduct a comprehensive analysis of the „constitutional law in action” of selected Member States, as expounded in constitutional case law, practice and texts. The functional method of comparative law will be used, analysing constitutional law as it functions in practice. Also, these themes will be analysed in an interdisciplinary fashion, taking account of other disciplines. It is this method which makes this research innovative: the national lines will be crossed, and system-neutral themes be used as a starting point, in order to formulate common principles as well as identify national diversity. It will thus advance the scientific debate on constitutionalism in the EU, and contribute to embedding it in common constitutional traditions, leaving room, where necessary, for national constitutional diversity.