Most medical specialities have defined medically unexplained syndromes such as fibromyalgia, to categorize patients with prominent but unexplained symptoms. Other such syndromes include irritable bowel syndrome, chronic fatigue syndrome and atypical chest pain. In this chapter we present evidence to suggest that fibromyalgia is not a unique clinical entity, but shares much with these other syndromes. We use historical, clinical and epidemiological evidence to illustrate this idea. The historical data emphasize the essentially arbitrary way in which fibromyalgia developed. The clinical evidence shows the considerable overlap between patients with fibromyalgia and those with other unexplained syndromes. From an epidemiological perspective we emphasize the strong associations between symptoms such as myalgia and fatigue. We conclude by suggesting that fibromyalgia is one of many medically unexplained syndromes which have more similarities than differences between them.