AbstractAid in understanding issues surrounding the construct validity of fatigue including the distinction between pathological versus nonpathological fatigue. Fatigue is a universal symptom reported by individuals in the general population as well as by those suffering from different medical and psychological illnesses, including cancer, multiple sclerosis, chronic fatigue syndrome, depression, and anxiety. Chronic fatigue is a significant problem in many primary care settings, and the debilitating and prolonged nature of fatigue can pose significant economic consequences for society. Researchers have struggled to better assess and understand the etiology and classification of fatigue within different illness groups.