This paper analyzes the incentive properties of the standard and burden of proof for a finding of negligence, when evidence about injurers' behavior is imperfect and rests with the parties. We show that the `preponderance of evidence' standard used in common law, together with ordinary exclusion rules defining legally admissible evidence, provides maximal incentives for potential tort-feasors to exert care. This holds even though, following the occurrence of harm, litigants have unequal access to evidence and may distort information in adversarial procedures. Abstracting from litigation costs, the optimal assignment of the burden of proof is shown to follow from the principle underlying the standard of proof. Our main results are derived in a mechanism design framework, but we also consider implementation as a sequential equilibrium with the court as a player in the game.