Fibromyalgia, also called (primary) fibromyalgia syndrome or fibrositis (syndrome), is a chronic soft tissue pain syndrome characterized by the presence of widespread musculosceletal aching, tender points at characteristic sites, fatigue, and poor sleep. The etiology of this common and painful condition is incompletely understood, non-restorative sleep, which has been correlated with an electroencephalic abnormality called alpha-delta sleep (intrusion of alpha rhythms in the non-REM sleep EEG), may be important. Several observations suggest that serotonin is likely to mediate both pain and non-REM sleep, and serotonin deficiency in the brain has been suggested to be of pathogenetic significance in fibromyalgia syndrome. A lower than normal threshold for pain can often be observed. The newly proposed criteria for the classification of fibromyalgia syndrome (widespread pain in combination with tender points at 11 or more of 18 specific tender point sites) are important for the differential diagnosis and to compare results in international studies.