Illness behavior refers to the ways in which symptoms are perceived, understood, acted upon, and communicated and include facial grimacing, holding or supporting the affected body part, limping, using a cane, and stooping while walking. Illness behavior can be unconscious or conscious: In the former, the person is unaware of the mental processes and content that are significant in determining behavior; conscious illness behavior may be voluntary and conscious (the two are not necessarily associated). The first broad category of inappropriate illness behavior is defensiveness, which is characterized by denial or minimization of symptoms. The second category includes somatoform disorders, factitious disorders, and malingering and is characterized by exaggerating, fabricating, or denying symptoms; minimizing capabilities or positive traits; or misattributing actual deficits to a false cause. Evaluators can detect the presence of inappropriate illness behaviors based on evidence of consistency in the history or examination; the likelihood that the reported symptoms make medical sense and fit a reasonable disease pattern; understanding of the patient's current situation, personal and social history, and emotional predispositions; emotional reactions to symptoms; evaluation of nonphysiological findings; results obtained using standardized test instruments; and tests of dissimulation, such as symptom validity testing. Unsupported and insupportable conclusions regarding inappropriate illness behavior represent substandard practice in view of the importance of these conclusions for the assessment of impairment or disability.