The concept of mild cognitive impairment (MCI) was proposed by Petersen et al. (1997, 1999) as a nosologic entity referring to elderly persons with mild cognitive deficit and without dementia. MCI is widely used in studies as an intermediate stage between cognitive normalcy and dementia. MCI now appears, however, to be a heterogeneous clinical entity. The many sources of heterogeneity that have been pointed out include: heterogeneity in etiological factors (various types of degenerative lesions, vascular risk factors, psychiatric features, concomitant non-neurological diseases), in clinical symptoms, and in clinical course (with decline, stable, or reversible cognitive impairment). New clinical criteria have thus been proposed for use in research and in clinical practice: 1) cognitive complaint from the patient, family, or both, 2) report by the subject or reporter of a decline in cognitive or functional performance, relative to previous abilities, 3) cognitive disorders evidenced by clinical evaluation: impairment in memory or another cognitive domain, 4) cognitive impairment without any repercussions on daily life, even if the subject reports difficulties concerning complex daily activities, and 5) no dementia. Those new criteria, essentially clinical, may be better adapted to both clinical research and daily clinical practice. Biological and radiological markers will provide greater and more systematic support for diagnosis in the near future, particularly for early detection of Alzheimer's disease.