Fibromyalgia (FM), also known as fibromyalgia syndrome (FMS) and fibrositis, is a common form of nonarticular rheumatism that is associated with chronic generalized musculoskeletal pain, fatigue, and a long list of other complaints. Some have criticized the classification of FM as a distinct medical entity, but existing data suggest that individuals meeting the case definition for FM are clinically somewhat distinct from those with chronic widespread pain who do not meet the full FM definition. Clinic studies have found FM to be common in countries worldwide; these include studies in specialty and general clinics. The same is true of general population studies, which show the prevalence of FM to be between 0.5% and 5%. Knowledge about risk factors for FM is limited. Females are at greater risk, and risk appears to increase through middle age, then decline. Although some authors claim that an epidemic of FM has been fueled by an over-generous Western compensation system, there are no data that demonstrate an increasing incidence or prevalence of FM; moreover, existing data refute any association between FM prevalence and compensation. Claims that the FM label itself causes illness behavior and increased dependence on the medical system also are not supported by existing research. This article reviews the classification, epidemiology, and natural history of FM.