SummaryThe overvalued idea, first described by Wernicke, refers to a solitary, abnormal belief that is neither delusional nor obsessional in nature, but which is preoccupying to the extent of dominating the sufferer's life. Disorders conforming to his definition are well documented, though their recognition as such has been variable, and they may not be as rare as is often thought. As well as sharing a distinctive phenomenology, the conditions develop in similar settings and carry a uniformly poor prognosis. Their pathogenesis is obscure and difficult to account for in conventional terms.