Dementia is a more severe and persistent cognitive impairment. Early in dementia, near memory impairment occurs, the ability to learn new things is significantly diminished, and those with severe dementia cannot even find their way home. With further development of dementia, distant memory is also impaired, thinking is slow and impoverished, comprehension and judgment of general things are getting worse, concentration is poor, and personality changes, such as impulsivity and childish behavior, occur. Emotional changes in dementia patients are manifested as anxiety, irritability, depression and emotional instability, sometimes manifested as emotional indifference, and delusions and hallucinations may also occur. In the late stages of dementia, patients are unable to take care of themselves and gradually lose their motor functions, and even need assistance in dressing, bathing, eating, and urinating and defecating. There are many causes of dementia, including degenerative diseases of the central nervous system, cerebrovascular lesions, tumors, chronic subdural hematomas, encephalitis, meningoencephalitis, neurosyphilis, AIDS, traumatic brain injury, endocrine disorders, liver failure, renal failure, pulmonary failure, electrolyte disorders, poisoning, and hypoxia. The examination includes cerebrospinal fluid examination, imaging, electrophysiological examination and neuropsychological assessment. The treatment of dementia should first clarify the cause of the disease, and early diagnosis and early treatment should be achieved.