an individual who is qualified to testify on scientific, technical, or professional matters and provide an opinion concerning the evidence or facts presented in a court of law. Eligibility to testify as an expert witness is based on the person’s special skills or knowledge as judged by the court. In U.S. federal courts, eligibility criteria are established by the Federal Rules of Evidence 702–706; see Daubert test. Mental health professionals often serve as expert witnesses in such cases as those involving insanity pleas and child custody.