Acute mental disturbance associated with physical illness is well described in early medical literature, but it was not until 1 AD that Celsus coined the term ‘delirium’ (Lindesay, 1999). Although delirium has many synonyms that are applied in particular clinical settings (Box 1), all acute disturbances of global cognitive functioning are now recognised as ‘delirium’, a consensus supported by both ICD–10 (World Health Organization, 1992) and DSM–IV (American Psychiatric Association, 1994) classification systems. Delirium is a complex neuropsychiatric syndrome that typically involves a plethora of cognitive and non-cognitive symptoms, resulting in a broad differential diagnosis dominated by mental disorders. Psychiatrists' skills in assessing cognitive function and psychopathology, coupled with their knowledge of psychotropic agents, make them well suited to improving detection, coordinating management and facilitating research into this understudied disorder.