Delirium is a common yet under-detected syndrome characterized by acute deterioration in mental status. Patients show attentional deficits and commonly also display altered levels of consciousness and psychotic features. The mechanisms remain poorly understood. Delirium is associated with multiple adverse outcomes including patient and carer distress, increased length of stay, long-term cognitive impairment, new institutionalization and death. Old age and dementia are the main predisposing factors, but there are a number of modifiable risk factors that are important within hospital environments. Treatment involves identifying acute causes and optimizing conditions for the brain. Distress should be detected and managed. One-third of delirium is preventable through a targeted multicomponent set of actions.