Mild cognitive impairment (MCI) refers to the transitional state between the cognitive changes of normal aging and very early dementia. MCI has generated a great deal of research from both clinical and research perspectives. Several population- and community-based studies have documented an accelerated rate of progression to dementia and Alzheimer's disease in individuals diagnosed with MCI. Clinical subtypes of MCI have been proposed to broaden the concept and include prodromal forms of a variety of dementias. An algorithm is presented to assist the clinician in identifying subjects and subclassifying them into the various types of MCI. Progression factors, including genetic, neuroimaging, biomarker, and clinical characteristics, are discussed. Neuropathological studies indicating an intermediate state between normal aging and early dementia in subjects with MCI are presented. The recently completed clinical trials as well as neuropsychological and nutritional interventions are discussed. Finally, the clinical utility of MCI, and directions for future research are proposed.