Delirium has been recognized since ancient times and was first called delirium by Celsus in the first century AD. Variously termed delirium, phrenitis and lethargy, severe disturbances of thought, mood and action were often seen to be associated with physical disease. In the nineteenth century delirium developed into a rather broader description of confusion, a state which emphasized chaotic thinking and cognitive failure but which included what we now call dementia as well as confusion arising in the context of other functional psychoses. During the twentieth century the concept of delirium has continued to evolve. Kurt Schneider emphasized clouding of consciousness as central to delirium and this assumption has largely persisted. Delirium has developed as an acute condition, often precipitated by major medical illness. Increased interest in elderly people as a special group has led to the recognition that delirium is a frequent feature of acute medical admissions of elderly patients.