Delirium is a serious complication that commonly occurs in critically ill patients in the intensive care unit (ICU). Delirium is frequently unrecognized or missed despite its high incidence and prevalence, and leads to poor clinical outcomes and an increased cost by increasing morbidity, mortality, and hospital and ICU length of stay. Although its pathophysiology is poorly understood, numerous risk factors for delirium have been suggested. To improve clinical outcomes, it is crucial to perform preventive measures against delirium, to detect delirium early using valid and reliable screening tools, and to treat the underlying causes or hazard symptoms of delirium in a timely manner.