An expert witness provides the prosecuting or defense attorney in a criminal trial, civil litigation, or arbitration with expert testimony on a specific aspect of a case. Law firms or prosecutors also consult with experts when they are developing a case. Your responsibilities may be to testify in court about medical evidence, scientific findings, or questions of criminal psychology, to name just a few specializations. In some instances, expert witnesses read statements for a deposition reviewed by an arbitrator or judge. As an expert witness, your duties are to review the case before testifying and provide clear expert evidence that is understandable to people outside of your field.