This chapter focuses on the Common Foreign and Security Policy (CFSP). It aims at the construction of explanatory theory following the ideas of J. Dougherty and R. Pfalfzgraff who argues that theory helps us to discover new knowledge most efficiently. The chapter also focuses on explanation of the factors driving security integration in Europe since 1989. It examines the aspects of the development of the CFSP, which support the assumptions made about the aspects. The aspects are intergovernmentalism, the impact of the international system and the impact of internal integrative dynamics. Relevant theories will be selected for each part of the explanation, contrasting the findings from the super-systemic level with the lower levels will indicate the limits of member state influence on the CFSP. In CFSP, agency from member governments is more important in the case of decisions on defence although agency from other sources such as the Commission may help to shape the integration process over time.