Necessity and proportionality hold a firm place in the international law governing the use of force by states, as well as in the law of armed conflict. However, the precise contours of these two requirements are uncertain and controversial. The aim of Necessity and Proportionality in International Peace and Security Law is to explore how necessity and proportionality manifest themselves in the modern world under the law governing the use of force and the law of armed conflict, and how they relate to each other. The book explores the ways in which necessity and proportionality are applied in practice and addresses pressing legal issues in the law on the use of force, including the controversial "unwilling and unable" test for the use of force in self-defense, drones and targeted killing, the application of this legal regime during civil war, and the need for further transparency in states' justification for the use of force in self-defense. The analysis of the role of military necessity within the law of armed conflict on the modern battlefield focuses on the history and nature of the principle of military necessity, the proper application of the principle of proportionality, how commanders should account for mental harm in calculating proportionality, and the role artificial intelligence and autonomous weapons systems may play in proportionality analysis. The book concludes with a discussion of the potential role of proportionality in the law governing post-conflict contexts.