Delirium is a common psychiatric disorder and occurs in many hospitalized older patients and has serious consequences including increased mortality rate. Despite its importance, health care clinicians often fail to recognize delirium or misdiagnosed as other psychiatric illness. Awareness of the etiologies and risk factors of delirium should enable clinicians to focus on patients at risk and to recognize delirium symptoms early. To improve early recognition of delirium, emphasis should be given to terminology, psychopathology and knowledge regarding clinical rating scale for delirium in the specific medical and surgical clinical settings. In this study, authors introduce rating scales for delirium and knowledge of clinical diagnostic process for delirium and give rise to appropriate assessment of delirium in the clinical situation.