One of the innovations of the Treaty of Lisbon has been the renaming of the European Security and Defence Policy (ESDP): since December 2009 the EU does not only have a Common Foreign and Security Policy (CFSP), it also has a Common Security and Defence Policy (CSDP). When the Treaty of Lisbon entered into force, this 'upgrade' have been a mere symbolic act, as EU member states differ in security and defence orientation, security and defence interests and security and defence policy. This work analyses how likely it is that the EU will forge and establish a common policy of substance, how feasible a genuine Common Security and Defence Policy is in view of national diversity. It does so by examining how different EU member states actually are with regard to security and defence issues. The first analysis chapter analyses the legal dimension of ESDP by comparing the national military law systems of EU member states and by answering the question to what extent the national constitutions and military laws impede, facilitate or encourage integration. The second analysis chapter analyses how crucial different national security and strategic cultures are for the creation of a genuinely common ESDP by discussing the following cleavages within the Union: Allies versus Neutrals; Atlanticism versus Europeanism; conscription versus professionalism. The third analysis chapter analyses how the political will and political preferences differ between EU countries by cpaturing the perceptions of key decision-makers and opinion-formers within national assemblies and parliaments in an online survey, both generally with regard to defence and security issues, and specifically in relation to ESDP. This work will show how national diversity - different national military law systems, different national security and strategic cultures and different national political preferences - shapes the development of a genuinely common ESDP,a nd will reveal the most likely face of a future CSDP.