Subjective cognitive impairment and mild cognitive impairment are the stage of the elderly before clinical development of dementia, in which memory function is specifically impaired. Though there were discussions that memory impairment of the elderly was a phenotype of physiological brain aging, it is now understood that those memory impairment should be interpreted as pathological phenotype of the diseased brain of the elderly. The memory and cognitive impairment of the elderly can be the target of research to prevent development of dementia. Brain reserve hypothesis and cognitive reserve hypothesis are discussed, which might open the new perspective to overcome the memory impairment of the elderly.