In this chapter, students will learn about the debates concerning security within liberal thought. The first section outlines traditional/Kantian liberalism. The second section introduces liberal economic thought regarding peace and war and the ideas of ‘douce commerce’. The third section describes the democratic peace thesis, and reviews the major discussions on the idea that liberal states do not fight wars with other liberal states. The last section outlines the major arguments in neo-liberal institutionalism. It concludes by highlighting the main differences between realist approaches to security and liberal approaches.