Delirium is common but hard to be detected in older persons, especially in patients with dementia. Inattention, disorganized thinking and acute changes or fluctuations in mental status are some of the typical presentations of delirium. Delirium is often multi-factorial and may be induced by a combination of factors related to patient, environment and medication. The hypotheses for the pathogenesis of delirium focus on the roles of neurotransmission, inflammation, acute stress and neuronal damage. Regular cognitive evaluation should be practiced in order to facilitate early detection and early intervention of delirium in older patients. Sometimes, delirium may lead to long-term cognitive impairment. Therefore, understanding how to prevent and manage delirium is essential for improving the quality of life in older persons.