Deterrence is invoked to justify a wide range of domestic and international policies and practices. Punishment is often justified as deterring both repeat offending and offending by others. The UK's (now-aborted) policy of deporting asylum seekers to Rwanda was largely defended as a means of deterring migrants from crossing the Channel. Recent strikes on Houthi rebels in Yemen were intended to deter both attacks on commercial vessels in the Red Sea and the expansion of the war in Gaza to other parts of the Middle East. The importance of deterring Russian aggression has underpinned the military support provided to Ukraine by its Western allies. Despite its central role in both international affairs and domestic politics, deterrence remains surprisingly under-theorised in the ethics of war and in moral and political philosophy more broadly. For example, although deterrence is often cited as a justification for war in political debates, it is largely neglected in the substantial body of work produced on the ethics of war in the last twenty years. And although there are significant bodies of work on deterrent punishment and nuclear deterrence, these seem ill-suited to evaluating the full range of domestic and international deterrent policies. This theoretical lacuna is troubling, given the importance, prevalence and often high stakes of deterrent policies. And the subject is challenging, since it engages complex deontological principles, such as the distinction between doing and allowing harm and the moral significance of using a person for the benefit of others. This workshop will bring together philosophers working on moral, legal, and political dimensions of deterrence to explore underlying theoretical issues that can help us assess the plausibility and legitimacy of deterrent justifications.