We review the most important concepts about delirium, from ancient times until the twentieth century. We also focus on the question of how these concepts have dealt with the particular problems posed by prognosis and outcome. Althought different terms have been used, a robust description of delirium has existed since antiquity--at some times as a symptom and at others as a syndrome. It is clear that, throughout the millennia, delirium has been--and still is--a highly lethal syndrome; a poor mental outcome for survivors was often noted. Not until the twentieth century was it thought that delirium was marked by a full recovery among survivors, and this was probably due to the desire for a clear distinction from dementia.