For centuries we've dismissed delusions as something for doctors to sort out behind locked doors. But delusions are more than just bizarre quirks—they hold the key to collective anxieties and traumas. In this history, Victoria Shepherd uncovers stories of delusions from medieval times to the present day, including that of France's King Charles VI, who thought he was made of glass; the dozens of Victorian women who were convinced that they were dead; and after the Battle of Waterloo, the epidemic of "Napoleons" admitted into asylums.