The principle of non-combatant immunity protects non-combatants against intentional attacks in war. It is the most widely endorsed and deeply held moral constraint on the conduct of war. And yet it is difficult to justify. Recent developments in just war theory have undermined the canonical argument in its favour – Michael Walzer's, in Just and Unjust Wars. Some now deny that non-combatant immunity has principled foundations, arguing instead that it is entirely explained by a different principle: that of necessity. In war, as in ordinary life, harms to others can be justified only if they are necessary. Attacking non-combatants, the argument goes, is never necessary, so never justified. Although often repeated, this argument has never been explored in depth. In this article, I evaluate the necessity-based argument for non-combatant immunity, drawing together theoretical analysis and empirical research on anti-civilian tactics in interstate warfare, counterinsurgency, and terrorism.