With wider and wider acceptance of mental health professionals in courts of law as expert witnesses, the tendency becomes very great to render expert opinions on a wide variety of topics. At times. the specific practitioner does not have the expertise to render opinions on these topics, and at other times the psychological instruments and methodology used do not allow such extrapolations and evaluations to be made. This paper discusses the ethical constraints in a variety of different areas of forensic practice, stressing how we must never go beyond the limits of our expertise.