Chronic fatigue syndrome (CFS) is the most common name^[1] given to a poorly understood, variably debilitating disorder or disorders of uncertain causation. It is also commonly known as myalgic encephalomyelitis or ME.

Symptoms of CFS include widespread muscle and joint pain, cognitive difficulties, chronic, often severe mental and physicalexhaustion and other characteristic symptoms in a previously healthy and active person. Fatigue is a common symptom in many illnesses, but CFS is a multi-systemic disease and is relatively rare by comparison.^[2] Diagnosis requires a number of features, the most common being severe mental and physical exhaustion which is "unrelieved by rest," is worsened by exertion, and is present for at least six months. All diagnostic criteria require that the symptoms must not be caused by other medical conditions. CFS patients may report additional symptoms,^[3] including muscle weakness, cognitive dysfunction, hypersensitivity, orthostatic intolerance, digestive disturbances, depression, poor immune response, and cardiac and respiratory problems. It is unclear if these symptoms represent co-morbid conditions or are produced by an underlying etiology of CFS.^[4] Full resolution occurs in only 5-10% of cases.^[5]

CFS is thought to have an incidence of 4 adults per 1,000 in the United States.^[6] For unknown reasons, CFS occurs more often in women than men, and in people in their 40s and 50s.^[7][8] The illness is estimated to be less prevalent among children and adolescents,^[5] but studies are contradictory as to the degree.^{[citation needed]}There is no medical test which is widely accepted to be diagnostic of CFS. It remains a diagnosis of exclusion based largely on patient history and symptomatic criteria, although a number of tests can aid diagnosis.^[9]

Whereas there is agreement on the genuine threat to health, happiness, and productivity posed by CFS, various physicians groups, researchers, and patient activists promote different nomenclature, diagnostic criteria, etiologic hypotheses, and treatments, resulting in controversy about many aspects of the disorder. The name CFS itself is controversial, as advocacy groups as well as some experts feel it trivializes the illness and have supported efforts to change it. Many alternative names for chronic fatigue syndrome exist.