As institutionalists have assumed, institutions are supposed to shape the behaviours of actors towards collective objectives and better prospects. For this purpose, the EU has established an institutionalised framework for developing security and defence integration. This institutionalised framework not only provides ‘rules of games’ for conducting common security and defence affairs, but is supposed to make member states become socialised and embedded in European integration. However, the role of member states cannot be ignored. In fact, the institutionalisation process from the EPC to the CSDP is affected by institutionalism and intergovernmentalism. In other words, although the CSDP has an institutionalised framework which offers rules and procedures for member states and EU institutions to implement a common security and defence policy, it also operates on the basis of intergovernmental co-operation. The different effect of institutionalism and intergovernmentalism can also be discovered through analysing the very distinct attitudes of Britain and Germany in this institutionalisation process. This thesis aims to investigate the institutional development and practice of the security and defence integration of the EU, and examine the role of member states in the process. Britain and Germany are chosen as comparative cases because these two countries have presented quite different preferences and attitudes towards the developments of European integration. This thesis concludes that although the institutionalisation process can affect member states in structuring behaviours and national interests, their political will is nevertheless the most important key to determining whether an institutionalised CSDP can fulfil the collective end of security and defence integration in the EU.