The case of John versus Im (2002) stands for the proposition that clinical neuropsychologists are not qualified to diagnose traumatic brain injury. This ruling by the Supreme Court of Virginia prohibits neuropsychologists from testifying about these professional conclusions in the courtroom. However, in clinical practice neuropsychologists are often asked to disentangle the relative contribution of brain dysfunction and psychological factors to presenting symptomology. In the proposed submission, the authors provide an analysis of the neuropsychological testimony at issue in John versus Im using the admissibility standards for expert testimony that were established and refined by a trilogy of cases from the Supreme Court of the United States. The paper provides support for the notion that neuropsychological method has an established scientific base of knowledge, standards for clinical competence, and evidence of peer-reviewed acceptance by medical related disciplines. No other scientific discipline has employed a more rigorous methodology for assessing cognitive function and disentangling the relative contributions of brain dysfunction and psychological factors to presenting symptomology. By limiting the testimony of neuropsychologists as to cause of an individual's cognitive impairment, courts will exclude opinions based on scientific research and specialized knowledge that would assist in the trier of fact.