The Supreme Court, based on three decisions over the past decade, now requires judges to examine the underlying basis of all testimony to ensure that only expert testimony supported by valid methods if inquiry is introduced as evidence in litigation. Under these standards, expert testimony in the courtroom, including medical testimony, is supposed to meet the same standards of intellectual rigor that professionals use outside the courtroom. If expert testimony does not meet this standard, the courts are expected to exclude the testimony and may dismiss the case without trial. In this article, the authors review cases that illustrate inconsistencies in the courts' approach to medical expert testimony. They argue that while there may be good reasons to require evidence of a higher quality and quantity that a physician would require in ordinary clinical decision making, as some courts have done, this practice is not faithful to the mandate of the Supreme Court. -- -- Reprinted from JAMA, September 18, 2002--Volume 288, No. 11 (288 JAMA 1382).