Fatigue is defined as a condition or phenomenon of declined ability and efficiency of mental and/or physical activities caused by excessive mental or physical activities, or diseases or syndromes. Acute fatigue is a normal condition that disappears after a period of rest ; in contrast, chronic fatigue does not disappear after an ordinary rest. Chronic fatigue impairs daily activities and contributes to various medical conditions and death. In addition, many people complain of chronic fatigue. It would thus be of great value to clarify the mechanisms underlying chronic fatigue and to develop efficient treatment methods to overcome it. Here, we review data primarily from biochemical, neuroimaging, and molecular imaging studies related to the neural and molecular mechanisms underlying chronic fatigue. We propose that repetitive and prolonged overwork and/or stress cause neural damage of a facilitation system as well as central sensitization and classical conditioning of an inhibition system. We also propose a new treatment strategy for chronic fatigue on the basis of its underlying neural/molecular mechanisms.