An evaluator's failure to recognize illness behavior and to adjust for its effect on symptom presentation can lead to erroneous conclusions regarding impairment or disability; here the authors review the associated phenomena of somatization, disorders that arise from it, and factors that contribute to somatization. Somatization is an individual's unconscious use of the body or bodily symptoms for psychological purposes or psychological gain. As the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Sixth Edition, notes, somatization is a “tendency to experience and report somatic complaints (physical symptoms) in response to psychosocial stress and seek health care services for them.” The AMA Guides also advises evaluators that “exclusion or inclusion of somatization disorder, factitious disorder, and/or malingering must be done with care.” A table lists developmental factors that may dispose to somatization (eg, somatizers are present in the family of origin, or coping mechanisms other than illness behavior are absent or unacceptable); a second table compares disorders that may involve somatization. Somatization is prevalent in clinical practice and is likely to occur with equal, or higher, frequency in the evaluation setting. The presence of somatization distorts symptom presentation and can exert a confounding effect on impairment assessment methodologies that rely on the accuracy of patients’ self-presentations.