Physicians who use the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) often serve as medical expert witnesses. In workers’ compensation cases, the expert may appear in front of a judge or hearing officer; in personal injury and other cases, the physician may testify by deposition or in court before a judge with or without a jury. This article discusses why medical expert witnesses are needed, what they do, and how they can help or hurt a case. Whether it is rendered by a judge or jury, the final opinions rely on laypersons’ understanding of medical issues. Medical expert testimony extracts from the intricacies of the medical literature those facts the trier of fact needs to understand; highlights the medical facts pertinent to decision making; and explains both these in terms that are understandable to a layperson, thereby enabling the judge or jury to render well-informed opinions. For expert witnesses, communication is everything, including nonverbal communication that critically determines if judges and, particularly, jurors believe a witness. To these ends, an expert medical witnesses should know the case; be objective; be a good teacher; state opinions clearly; testify with appropriate professional demeanor; communicate well, both verbally and nonverbally; in verbal communications, explain medical terms and procedures so listeners can understand the case; and avoid medical jargon, finding fault or blaming, becoming argumentative, or appearing arrogant.