According to the American College of Rheumatology the diagnosis of fibromyalgia is based on criteria for the classification of fibromyalgia consisting entirely of clinical signs and symptoms. For diagnostic reasons autonomic disturbances and mental features have to be considered. The distinction between fibromyalgia (tender points) and myofascial pain syndrome (trigger points) is essential. Internal and neurological disorders as a primary cause of fibromyalgia have to be excluded. The etiology and pathogenesis of fibromyalgia still remain uncertain. The myopathological patterns in fibromyalgia are non-specific: type II fiber atrophy, an increase of lipid droplets, a slight proliferation of mitochondria, and a slightly elevated incidence of ragged red fibers. Initial reports on some allelic abnormalities in the serotonin system seem to highlight the important role of serotonin already presumed earlier. Significantly high levels of substance P in the cerebrospinal fluid of FM patients additionally support the impact of these neurotransmitters on both nociceptive and antinociceptive mechanisms.