Delirium is an acute neuropsychiatric disturbance secondary to a pathophysiological insult, including illness, injury, or the effects or withdrawal of drugs. Features are variable, and fluctuate, with drowsiness or inattention, change in cognition, and often delusions, hallucinations, emotional, autonomic, and motor disturbance. It is commoner in those with prior dementia, and is seen in medical, surgical, intensive care, and palliative settings. Pathological mechanisms are uncertain. Nonpharmacological prevention strategies reduce incidence by up to 30%, including early mobilization, drug review, and sleep hygiene. Diagnosis is clinical, supported by various tools. Delirium is often underdiagnosed. Treatment comprises identifying and treating the (sometimes multiple) causes, avoiding complications, rehabilitation, and symptomatic relief of distress. Short-term, low-dose antipsychotic drugs may be used, but with limited supporting evidence. Prognosis is variable: some with delirium resolve within 24 hours, one-quarter persist for three months, and some never recover. Morality is up to 40% at six months.