An impairment evaluation aims to produce a report that is clear and consistent with the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Fourth Edition, which outlines the three-step process of medical evaluation, analysis of the findings, and comparison of the results with the impairment criteria. This article reviews common errors that can occur in each of these steps. Medical evaluation is the basis for the evaluation of impairment and relies on the patient's medical history; the latter must include adequate background information, specify data sources, document pre-existing status and specifics of the injury, and document history from onset to current status. The medical evaluation should support a report that explains the effects of the medical condition on life activities; whether the medical condition is stable; whether the individual is likely to suffer incapacitation, injury, harm, or further impairment; and whether restrictions or accommodations are warranted. Common errors in this step include determinations of permanency and stability, and raters should be aware that the AMA Guides is used to assess impairment, not disability. Forms from the AMA Guides are appropriate and help mitigate errors. An audit form is included and can help physicians produce consistent, efficient reports.