The burden of proof, a central feature of adjudication and other decision-making contexts, constitutes an important but largely unappreciated policy instrument. The optimal strength of the burden of proof, as well as optimal enforcement effort and sanctions, involves trading off deterrence and the chilling of desirable behavior, the latter being absent in previous work. The character of the optimum differs markedly from prior results and from conventional understandings of proof burdens. There are important divergences across models in which enforcement involves monitoring, investigation, and auditing. A number of extensions are analyzed, in one instance nullifying key results in prior work.