In order for a medical opinion to be admissible as evidence in civil, criminal and administrative cases, the basis of the opinion must fulfill either the Daubert Criteria or the Frye test, depending on the jurisdiction. The judge of the court rules on the admissibility of the expert opinion. The effect of Daubert has been to limit expert testimony to opinions which are based on a scientific foundation. Daubert specifies that adequate scientific support and method and a known error rate must exist. The testimony of a mental health expert rendering an opinion using criteria which does not meet Daubert standards is weakened by the implication that it is not based on "sound science." In some instances, for example, a mental health expert uses an approach where there are no peer-reviewed studies or methods, such as when psychologists compose their own neuropsychological test batteries. In most cases where an attorney is considering a "Daubert challenge," a contemporaneous and up-to-date literature search is indicated. Also, extensive case law presently exists as to specific issues. Being familiar with the Daubert criteria enhances effectiveness in challenging a mental health expert's opinion, whether on voir dire or cross examination. On direct examination, the strengths of an opinion reached under Daubert criteria become a "teaching moment" for the trier of fact, because it will be founded on the science of mental health assessment.