Chronic fatigue syndrome is characterized by debilitating fatigue that is not relieved with rest and is associated with physical symptoms. The Centers for Disease Control and Prevention criteria for chronic fatigue syndrome include severe fatigue lasting longer than six months, as well as presence of at least four of the following physical symptoms: postexertional malaise; unrefreshing sleep; impaired memory or concentration; muscle pain; polyarthralgia; sore throat; tender lymph nodes; or new headaches. It is a clinical diagnosis that can be made only when other disease processes are excluded. The etiology of chronic fatigue syndrome is unclear, is likely complex, and may involve dysfunction of the immune or adrenal systems, an association with certain genetic markers, or a history of childhood trauma. Persons with chronic fatigue syndrome should be evaluated for concurrent depression, pain, and sleep disturbances. Treatment options include cognitive behavior therapy and graded exercise therapy, both of which have been shown to moderately improve fatigue levels, work and social adjustment, anxiety, and postexertional malaise. No pharmacologic or alternative medicine therapies have been proven effective. (Am Fam Physician. 2012;86(8):741–746. Copyright © 2012 American Academy of Family Physicians.)