Alzheimer￠s disease (AD) is a neurodegenerative illness characterized by a profound impairment of cognitive function. Mild cognitive impairment (MCI) is characterized by a less severe level of cognitive impairment but with no dementia. MCI has been presumed to be a high-risk for developing dementia, especially AD. Actually, there have been disputes on definition and clinical outcome of MCI. Heterogeneity in the term MCI was demonstrated by both clinical and epidemiological researches. As the “pre-dementia” phase of AD draws more and more attention from researchers, it is essential to understand the changes in definition of MCI. This paper reviewed the changes in concept of MCI, and introduced the new diagnostic criteria of AD proposed by the National Institute (U.S.) Aging-Alzheimer’s disease workgroup.