The topic of this thesis is a theoretical analysis of the institutional design of the Common Foreign and Security Policy (CFSP) after the adoption of the Lisbon treaty. The main aim of the thesis is to reveal the reasons explaining a development towards centralization of the policy-making process of the CFSP that the Lisbon treaty introduced. Centralization is equated with a delegation of sovereignty from member states to a supranational level - High Representative for the CFSP/European External Action Service. The thesis for this purpose tests three possible explanations built upon three main theories of international relations: the centralisation as a result of the 1.decreasing EU's relative military power (neorealism) 2.deteriorating collaborative problems (neoliberalism) and 3.increasing degree of Europeanization of the nation-state identities and interests of the member states (constructivism) within covered time period from 2001 to 2007. The thesis applies a qualitative method, it is a case study. The conclusion of the thesis is that there are two reasons for the development towards more centralized policy-making process of the CFSP after the Lisbon treaty. Firstly, it is the decreasing EU's relative military power (neorealism) and secondly increasing level of Europeanization of nation-state...