Delirium is not developed naturally and is not an endogenous disease. However, delirium has occurred in some specific populations, such as the elderly or medically compromised patients. These become limitations on the systematic study of the epidemiology of delirium. Many epidemiologic studies of delirium have been focused on the elderly due to the fact there are relatively few in the general population. In addition, assessment tools with various sensitivity and specificity have been used. Advanced age, male, poorer cognitive and functional status, and alcohol abuse are well known risk factors of delirium. the epidemiological data will be reviewed in the elderly, patients with dementia, hip fractures, patients hospitalized in intensive care units, terminal cancer and patients with stroke.