This thesis aims to understand energy security behavior of the EU in the light of two IR theoretical stances: rationalism-neo-realism and constructivism. In order to do that, the study targets the question: “What is the central motivation of EU energy security policies- norms or interests- and how the different IR theories of rationalism-neo-realism and constructivism can interpret EU energy security policies differently”. Power and wealth being the most crucial, interest-based policies refer to the efforts that will promote the material interests, disregarding the normative considerations. Norm-based policies, on the other hand, represent the policies which are resulted from the identity, norms and political culture, and which are sometimes taken at the expense of the material interests. Thus, this thesis proposes two contradictory impulses- interests and norms- for EU energy security behaviour, and tries to understand which one is dominant in energy security decision-making process. Relying on official EU documents and on literature works, this study reaches to the conclusion that in environmental topics, the EU is strongly committed to the norms. However, in the topics considering the supply security such as guaranteeing the present and future energy supplies, and energy diversification- the EU follows more interest-based policies. In conclusion, although the EU is the most committed actor to environmental protection in the world, a fully normative approach is still lacking in EU energy security behavior.