Fibromyalgia is a common illness characterized by chronic widespread pain, sleep problems (including unrefreshing sleep), physical exhaustion and cognitive difficulties. The definition, pathogenesis and treatment are controversial, and some even contest the existence of this disorder. In 1990, the American College of Rheumatology (ACR) defined classification criteria that required multiple tender points (areas of tenderness occurring in muscles and muscle-tendon junctions) and chronic widespread pain. In 2010, the ACR preliminary diagnostic criteria excluded tender points, allowed less extensive pain and placed reliance on patient-reported somatic symptoms and cognitive difficulties. Fibromyalgia occurs in all populations worldwide, and symptom prevalence ranges between 2% and 4% in the general population. The prevalence of people who are actually diagnosed with fibromyalgia ('administrative prevalence') is much lower. A model of fibromyalgia pathogenesis has been suggested in which biological and psychosocial variables interact to influence the predisposition, triggering and aggravation of a chronic disease, but the details are unclear. Diagnosis requires the history of a typical cluster of symptoms and the exclusion of a somatic disease that sufficiently explains the symptoms by medical examination. Current evidence-based guidelines emphasize the value of multimodal treatments, which encompass both non-pharmacological and selected pharmacological treatments tailored to individual symptoms, including pain, fatigue, sleep problems and mood problems. For an illustrated summary of this Primer, visit: http://go.nature.com/LIBdDX.