In Daubert v. Merrell Dow Pharmaceuticals Inc. (1993), the U.S. Supreme Court swept aside the Federal standard of Frye v. United States, which allowed expert evidence that enjoyed "general acceptance" in the scientific community. The Court also held that Federal trial judges, in their role as "gatekeepers" for admitting evidence under the Federal Rules of Evidence, must ensure that expert testimony "is not only relevant, but reliable." To replace "Frye's" single test, the U.S. Supreme Court in "Daubert" identified four factors to guide judges in assessing expert testimony. First, can the expert's theory be tested? Second, has the analysis been subjected to peer review and publication? Third, are there governing standards, or known or potential error rates, for the expert's techniques? Fourth, has the relevant scientific community accepted the expert's theory or technique? In the 1999 "Kumho Tire" decision, the U.S. Supreme Court extended "Daubert" beyond scientific witnesses to all expert testimony that involves technical and other specialized knowledge. Veteran lawyers agree that "Daubert" has had a profound effect on trial courts, even though it is a fairly straightforward reading of the Federal Rules of Evidence. This article provides guidelines for selecting a "Daubert"-proof witness.