This research aims to analyze how constructivism has envisaged security and the contribution of constructivism to the security debate. We shall argue that constructivist and rationalist approaches to international security and the European Security and Defense Policy ESDP generate competing perspectives. In the first section of the paper, we briefly discuss diversified perspectives in constructivist thought and present the main assumptions of the constructivist approach. Secondly, emphasis shifts to presenting the contributions of constructivism to security studies. The constructivist challenge to the rationalist assumptions have been studied with emphasizing the differences on specific concepts such as power, anarchy, cooperation, capability, conflict and sovereignty. As a case study, this essay examines the constructivist understanding on the ESDP and presents the constructivist critiques to the rationalist approaches. Whilst security studies have been increasingly approached by constructivism with presenting ontological contributions, there is still need for further theoretical and empirical research