In the trial of products liability cases, representing catastrophically injured plaintiffs, counsel often consider the need to test a product and call upon expert witnesses to testify at trial to the safety performance or the inadequacy of a particular product as shown in testing. Long before a party considers running product tests, the legal evidentiary criteria for admissibility must be studied to assure counsel that these often extraordinarily expensive analyses will come into evidence. The admissibility of product tests is often dependent upon the purpose for which the results are being offered at trial. The following is a short list of the factual and legal predicate often proffered for the admissibility of such testing: to reconstruct the injury-producing injury; to illustrate or demonstrate scientific principles relevant to an understanding of aspects of the accident without reconstructing it; to dynamically study the performance of aspects of the product to gauge its safety; or to dynamically study the performance of an aspect of an alternatively designed product to gauge its feasibility or comparative safety. Courts have developed various criteria for allowing or disallowing product testing based upon which of these bases is relied upon.