Borrowing from the theoretical insights of Europeanization and the related literature of new institutionalism, this thesis investigates the political and institutional interaction between the British government and the European Security and Defence Policy (ESDP), an increasingly active and institutionalised area of European Union (EL') member state intergovernmental security and defence policy cooperation. Although academic literature has increasingly focused on theoretical and empirical examinations of the Europeanization of EU member state foreign policy in both individual and comparative frameworks, very little scholarly attention has been directed towards analysing these Europeanization dynamics in the specific policy area of security and defence. Recognising the central role of EU member state governments in shaping ESDP, this thesis conceives of the Europeanization of British security and defence policy as a dialectical process involving both the British government's attempts to project its national policy preferences to the EU level, as well as the domestic impact of its participation in ESDP institutions and operations over time. Through a careful empirical analysis of the political and institutional interaction between the British government and the EU-level political and institutional framework of ESDP utilising a process tracing approach, this thesis argues that over the course of the seven-year time period between December 1998 and December 2005 British security and defence policy underwent a gradual process of Europeanization. On one hand, it uncovers the British government's significant role in shaping ESDP during several key periods of its development including the run-up to Nice European Council and the negotiations over the European Constitutional Treaty. Conversely, it identifies the significant domestic impact of the British government's participation over time in the political and institutional framework of ESDP on the content and conduct of its security and defence policy, signalling a gradual top-down Europeanization of this policy area.