The article details the history, concept, definition and assessment of the still enigmatic condition of 'fibrositis' or, as it has more recently been called, 'fibromyalgia'. The concept and diagnosis became popular, especially in North America, in the 1970s, after the seminal publications of Hugh Smythe (1972) and Smythe and Moldofsky (1977). It is noticeable that there does not appear to be an early case report as there is for instance for gout, rheumatoid arthritis or certain vasculitides. This may be one reason why we still lack a commonly shared clinical image of the 'typical' case. After Smythe and coworkers, operational definitions and classification criteria were given by Yunus et al, Lautenschläger et al (both in 1989) and Wolfe et al in 1990. The latter received the endorsement of the American College of Rheumatology and are now the most widely used. They identify fibromyalgia as a musculoskeletal disorder with spontaneous widespread pain and exaggerated tenderness as prominent and distinctive features. The other two criteria sets refer to a different concept of fibromyalgia as a 'functional' or 'dysfunctional' disorder. These and other nosological differences pose problems for clinical as well as epidemiological research. They may be of minor importance if it is accepted that any present definition is arbitrary and that a wide range of possible elements are more relevant to research than a uniform concept of a disease called fibromyalgia.