An expert witness is a person with special knowledge who helps the fact-finder understand technical, clinical, or scientific issues. Depending on the type of case, an expert may or may not need personal knowledge of the facts of the case. In a sexual abuse case, the substantive evidence might be the findings of a physical examination, the child's disclosure statement to a social worker, lay testimony from the child, lay testimony from the child's mother, and expert testimony from a mental health professional. Another aspect of expert testimony involves the "rehabilitation" of a child witness's credibility after the defense attorney attacks it. Expert testimony provides a scientific analysis of age-related accuracy of memory and credibility. In outlining principles for expert testimony, one section of the chapter discusses the importance of honesty, evenhandedness, respect for the limits of current knowledge in the field of expertise, and preparation. Other sections of the chapter focus on who qualifies as an expert witness, the form of expert testimony, types of information on which expert witnesses may rely to form opinions offered in court, expert testimony based on scientific principles or techniques, and expert testimony in physical abuse and neglect cases. The latter testimony may pertain to medical opinion on the cause of injury, battered child syndrome, shaken baby syndrome, and Munchausen syndrome by proxy. Another section deals with expert testimony in sexual abuse litigation. Such expert testimony may pertain to medical evidence, the psychological effects of sexual abuse, child interview techniques, and the psychological profile of a sex offender. 83 references