The concept of delirium has a long and confusing history. This article outl
ines the development of ideas relating to core features of the syndrome: di
sturbance of consciousness, disturbance of cognition, its course and its ex
ternal causation, The modern concept of delirium, and the diagnostic criter
ia found in current classifications are based upon a long tradition of clin
ical observation in younger patients, and their emphasis on positive sympto
ms and identifiable external causes may not be applicable to our ageing pop
ulation.