Repetitive illness sometimes is wrongly called repetitive injury or cumulative trauma, but the latter are misnomers because the employee cannot identify a specific injury as a cause of the symptoms. In workers’ compensation, such gradual illness claims may be compensable if the condition arises during the course of employment, which requires that it be caused by occupational duties, exposures, or equipment used on the employer's premises. Expert impairment evaluators face three requirements: they must know the best scientific evidence currently available regarding causation of the condition(s) in question, ie, generic causation; the facts of the individual case, ie, specific causation; and the legal threshold in the applicable jurisdiction for acceptance of a condition as work related. The AMA Guides to the Evaluation of Disease and Injury Causation, Second Edition, is an excellent resource and provides the physician a blueprint for the assessment of causation in occupational injury and illness claims. The book adopts the methodology developed by the National Institute for Occupational Safety and Health and the American College of Occupational and Environmental Medicine. When asked to render opinions regarding causation, a physician is wise to consider this methodology in determining the work relatedness of the condition. Medical opinions based on an accepted methodology and the best scientific evidence will result in better patient outcomes.