In the Anglo-American legal system, rules of evidence developed incrementally via court opinions resting on the inherent power of courts to define their own procedures. Today, most of the rules in the United States are contained in comprehensive codes, usually enacted by legislatures. It is common in some jurisdictions to have medical experts testify to “a reasonable degree of medical certainty.” By extension, some judges demand that scientists testify to “a reasonable degree of scientific certainty.” For centuries, Anglo-American law did not distinguish one type of expert testimony from another. On the surface, a uniform standard governed the admission of the testimony of all qualified experts. Since the early 1970s, the Frye standard was subjected to increasingly critical analysis, limitation, modification, and finally, outright rejection. The adoption of the Federal Rules of Evidence in 1974 intensified the retreat from Frye.