Delirium is an acutely occurred and transient brain dysfunction characterized with cognitive dysfunction, lowered consciousness, disturbed attention, abnormal psychoactivity, and disordered sleep-wake rhythm. Delirium is frequent in mechanically ventilated patients in the intensive care unit (ICU), with reported incidences up to 70% or 87%. Possible reasons leading to delirium include the intervention-heavy environment of the ICU, mechanical ventilation and sleep disturbances. The occurrence of delirium is associated with worse outcomes including prolonged mechanical ventilation and ICU stay, increased complications, prolonged hospital stay, higher mortality, and elevated medical expenses. It is also associated with worse long-term outcomes including declined cognitive function, lowered life quality, and elevated long-term mortality.