The number of patients with dementia is rapidly increasing, and it has become an important issue not only in medical care but also in society. As a countermeasure against dementia, prevention of dementia has been emphasized. Until recently, efforts have been made to improve lifestyle habits for the prevention of dementia. In Finnish Geriatric Intervention Study to Prevent Cognitive Impairment and Disability (FINGER), multidomain intervention of diet, exercise, cognitive training and vascular risk monitoring prevent cognitive decline in at–risk elderly people. In Lancet commission, modifying 12 risk factors was stated for the prevention of dementia. In World Health Organization guidelines, the 12 interventions for the risk reduction of cognitive decline and dementia were assessed. Recently, the disease–modifying therapies which affect the pathophysiological mechanisms in Alzheimer disease have been developed. The monoclonal antibodies, aducanumab, lecanemab and donanemab that selectively bind for amyloid β protein were effective for the prevention against the progression in early Alzheimer disease in phase 2 or 3 trials. Aducanumab and lecanemab have been approved by the US Food and Drug Administration. These new disease–modifying therapies might be promising preventive method in future.