The Thesis provides an examination of the multidimensional phenomenon of the European Union and then makes an inquiry on the development of the Community’s energy policy focusing on the European Commission’s objectives and the national preferences of the Member States. With the utilization of conceptual tools found in the theoretical toolbox of the paradigm of Defensive Realism this study attempts to answer the research question, “To what extent can a common EU energy policy be developed?” The purpose is to explain the phenomenon of the EU through a defensive realist prism, to analyze and discuss the constraints and the prospects concerning the creation of a common energy policy, the behavior of the member states in general and of the EU3 particular, namely of the United Kingdom, of France and of Germany. The main conclusion is that all member states, both the powerful and the weaker ones, are confronted with some common challenges. Although they do have different energy mixes that lead them to make distinctive and separate energy policy choices, while often have national preferences and interests that are conflicting with each other, simultaneously they do share similar energy supply and national security concerns such as their increasing import dependence, their exposure to high and volatile oil prices and the international competition for resources. Since they behave as defensive realist states and as energy security seekers it is an imperative for their survival and well-being, apart from their individual policies and bi-lateral agreements, to allow on the one hand the development of an internal energy market and on the other the external dimension of an EU energy policy. Therefore they benefit from the size of an EU-wide energy market and from the regulatory frameworks, the dialogues, the partnerships and the other initiatives that their institutional creation promotes. In conditions of growing multi-polarity within the international system, which thankfully at present has a balanced form, acting together can be a strategic option for survival and security. Consequently the main conclusion is that although the member states will continue to operate as independent actors within the anarchical global arena the cost of not utilizing the scale that the EU offers will probably be unbearable.