The burden of proof is a central feature of all systems of adjudication, yet one that has been subject to little normative analysis. This Article examines how strong evidence should have to be in order to assign liability when the objective is to maximize social welfare. In basic settings, there is a tradeoff between deterrence benefits and chilling costs, and the optimal proof requirement is determined by factors that are almost entirely distinct from those underlying the preponderance of the evidence rule and other traditional standards. As a consequence, these familiar burden of proof rules have some surprising properties, as do alternative criteria that have been advanced. The Article also considers how setting the proof burden interacts with other features of legal system design: the determination of enforcement effort, the level of sanctions, and the degree of accuracy of adjudication. It compares and contrasts a variety of legal environments and methods of enforcement, explaining how the appropriate proof requirements differ qualitatively across contexts. Most of the questions raised and answers presented differ in kind — as well as in result — from those in prior literature.