In a medical malpractice action, scientific testimony from expert witnesses, usually a physician who is a specialist in the relevant field, is often critical to a successful outcome, whether for the negligently injured patient or the wrongly accused doctor. For many years, the testimony of an expert witness offering such evidence would be admitted at trial if the opinion offered was based on “a well-recognized scientific principle or discovery” that was “sufficiently established to have gained general acceptance in the particular field in which it belongs.”1 That standard changed in 1993 with the United States Supreme Court's decision in Daubert v. Merrell Dow Pharmaceuticals, Inc .2 In this article we provide an introduction to the Daubert standard, with particular emphasis on the concept of “relative risk” that courts are using with increasing frequency to determine issues of causation in medical malpractice cases.3 Interpreting Rule 702 of the Federal Rules of Evidence ,4 which provides that in cases where the evidence to be provided requires “scientific, technical, or other specialized knowledge,” the testimony must be based on “sufficient facts or data,” and it must be “the product of reliable principles and methods” applied reliably to the facts of the case, the Daubert court implemented a flexible inquiry, requiring trial judges to act as gatekeepers who screen expert testimony to ensure that it is both reliable and relevant before it is presented to a jury. Knowing what factors trial judges will consider in determining admissibility will greatly aid expert witnesses in preparing their opinions and in advising lawyers and their clients when a particular opinion sought is not scientifically sound. “[E]ven a qualified expert is capable of rendering … Address correspondence to John T. Sartore, JD, Paul Frank + Collins PC, One Church St, PO Box 1307, Burlington, VT 05402-1307. E-mail: jsartore{at}pfclaw.com