Delirium is a common disorder in ill older patients, characterized by a fluctuating disturbance of consciousness and changes in cognition that develop over a short period of time. Studies have shown that delirium is an independent predictor of increased length of hospital stay, and is associated with increased dependency and mortality, as well as being distressing for patients and families. Much is known about the epidemiology of delirium, including predisposing factors such as pre-existing dementia and advanced age, and common precipitants such as infection, drugs and major surgery. In comparison, very little is known about the neuropathological mechanisms that lead to the development of delirium.