Delirium is a common mental disorder with serious adverse outcomes in hospitalised patients. It is associated with increases in mortality, physical morbidity, length of hospital stay, institutionalisation and costs to healthcare providers. A range of risk factors has been implicated in its aetiology, including aspects of the routine care and environment in hospitals. Prevention of delirium is clearly desirable from patients' and carers' perspectives, and to reduce hospital costs. Yet it is currently unclear whether interventions for prevention of delirium are effective, whether they can be successfully delivered in all environments, and whether different interventions are necessary for different groups of patients.