In 1993, the Supreme Court’s opinion in Daubert v. Merrell Dow Pharmaceuticals ushered in a new era with regard to the admissibility of expert testimony. As expert testimony has become increasingly essential in a wide variety of litigated cases, the Daubert opinion has had an enormous impact. If plaintiffs’ expert proof is excluded on a crucial issue, plaintiffs cannot win and usually cannot even get their case to a jury. This discussion begins with a brief overview of the Supreme Court’s three opinions on expert testimony—often called the Daubert trilogy—and their impact. It then examines a fourth Supreme Court case that relates to expert testimony, before turning to a variety of issues that judges are called upon to resolve, particularly when the proffered expert testimony hinges on scientific knowledge.