Establishing a valid and reliable impairment rating for persons with mental and behavioral disorders is particularly challenging. This article focuses on the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Fourth Edition, Chapter 14, Mental and Behavioral Disorders; because of similarities between the fourth and fifth editions, most comments here apply as well to the fifth edition. The section regarding mental and behavioral disorders is based in the US Social Security Administration (SSA) disability determination. The AMA Guides measures impairment in the same four areas of functioning designated by SSA: activities of daily living; social functioning; concentration, persistence, and pace (ie, focused attention); and deterioration or decompensation in work-like settings (“adaptation”). The assessment of severity of impairment is based on the five-level SSA approach and is applied to each of the four areas of functioning. The authors define and interpret the practical meanings of terminology in the AMA Guides associated with classes of severity: no impairment (self-explanatory); mild impairment (compatible with most useful functioning); moderate impairment (compatible with some but not all useful functioning); marked impairment (significantly impedes useful functioning); and extreme impairment (not compatible with useful functioning). Impairment ratings are truly applicable only when clinicians can approach the process in a consistent manner using consistent terminology and methodology; the latter two will improve both the understanding of the rating process and its reliability and validity.