The question of self-defence is a central concept in war theory and international criminal law, but how is this relevant in EU law? Security questions and the law of war are seemingly far from the traditional area of free movement in EU law. Yet the Russian full scale invasion of Ukraine in 2022 and the current security situation in Europe have put the question of self-defence high on the agenda. This chapter examines whether the EU has its own collective self-defence mechanisms by discussing the solidarity clause in Article 222 TFEU, as well as the mutual assistance provision 42 (7) TEU. Further, the chapter discusses how the Kantian legacy has shaped the rules of self-defence in the UN Charter and how it can help us understand the boundaries of using self-defence in EU law.