Fatigue is probably the most common symptom from both acute and chronic illnesses. It can be defined as a pervasive sense of tiredness or lack of energy that may not be related to exertion. Fatigue is transitory in most cases, however, if it is prolonged or disabling, a significant problem such as chronic fatigue syndrome (CFS) may be warranted. CFS is a complex, debilitating disorder characterized by severe persistent or relapsing fatigue for at least 6 months and a group of characteristic but nonspecific symptoms. Many researchers have proposed that CFS has a specific underlying cause. Currently, however, there is no evidence that supports this view. In addition, since there is no specific physical findings or definitive laboratory tests for consistent biological markers, the diagnosis of CFS depends on operational criteria that do not afford validity and is primarily a diagnosis by exclusion. The prognosis is poor and often the disability and impairment of daily function and performance may be prolonged. The limited understanding of CFS has complicated the management of this disorder. Therefore, the treatment of CFS may be variable and should be tailored to each patient-it should include exercise, diet, good sleep hygiene, antidepressants, and other medications, depending on the patient's clinical presentation.