Ethical reflections on war—and the positive laws these reflections have inspired—have framed their undertaking as the effort to limit and regulate state violence. Ethical reflections on punishment have not been framed in the same way, but they should be. Three characteristics of the philosophy (and laws) of war prove especially instructive for the philosophy (and laws) of punishment. First, the ethics of war is an ethics of violence: it acknowledges and addresses the gritty and often brutal realities of actual armed conflict. Punishment theory too often denies the violence of punishment or otherwise neglects the realities of penal practices. Second, philosophers of war tend to keep the usual agent of war’s violence—the state—squarely in view, whereas punishment theory tends to focus on the target of punishment rather than its agent. Third, and most importantly, commentators on the ethics of war have come to realize that the humanitarian project of limiting violence is a different and more difficult task than the project of justifying violence. This insight has produced the jus in bello: a set of principles aimed at limiting the violence of war without adopting a view of the war’s justification. Punishment theory has long been focused on the project of justifying punishment, but this Article sketches the contours of a jus in poena: philosophical and legal principles designed to regulate the conduct of punishment without adopting any particular theoretical justification for punishment.