The most important antitrust penalties are treble damage awards based on the individual harms that violations cause. For these penalties to function as an economically rational deterrent, there must be a practical mechanism for proving individual harm, and for distinguishing such harm from speculation. In this article, the authors present an account of that mechanism. First, they argue that the law\u27s measure of antitrust damages is based on a standard of net individual harm that is qualified in certain cases by a principle of net social harm. Net harm is measured by the difference between the plaintiff\u27s actual condition (given that the violation has occurred) and its but-for condition (assuming that the violation did not occur). Second, the authors show that law\u27s mechanism for proof of damages requires a projection of the but-for condition from a reasonably comparable base experience. The projection must use both the evidentiary foundation and a theoretical model to isolate the defendant\u27s illegal conduct as the difference between the actual and but-for conditions. Proof is speculative if the projection fails to account for actual or theoretical factors other than the violation that may have caused the asserted harm
