The AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Fourth Edition, introduces a new system of rating lower extremity impairments that allows use of more than one method for arriving at a rating. Such flexibility allows the rater greater leeway to assess each patient's unique status but requires the clinician to clearly understand the diagnosis, pathoanatomy, and expected sequelae of a condition. For example, diminished muscle function can be evaluated in four ways (gait, atrophy, weakness, and peripheral nerve injury), but impairments should be estimated under only one of these criteria. Tables in the AMA Guides give impairment values for the whole person, as well as for the lower extremity and the part, where applicable. Impairments can be calculated in several broad categories, including the following: limb length discrepancy; gait derangement; unilateral atrophy; manual muscle testing; range of motion measurements and ankylosis; arthritis; amputations; skin loss; diagnosis-based estimates; peripheral nerve; vascular disorders; and causalgia and reflex sympathetic dystrophy. Each category includes general guidelines that help raters decide when to use that specific section. In addition to clarifying and discussing the categories, the article provides references to specific sections and tables in the AMA Guides, Fourth Edition.