The concept of mild cognitive impairment (MCI) has been proposed by Petersen et al. (1997, 1999) as a nosologic entity referring to elderly persons with mild cognitive deficit without dementia. MCI is being widely used in studies as an intermediate stage between cognitive normalcy and dementia. However, MCI appears to be a heterogeneous clinical entity. Multiple sources of heterogeneity have been described: heterogeneity in aetiological factors (various types of degenerative lesions, vascular risk factors, psychiatric features, association of pathological conditions), heterogeneity in clinical symptoms, and heterogeneity in clinical course with decliners and non decliners presenting stable or reversible cognitive impairment. Thus new criteria of MCI are proposed for use in research but also in clinical practice. MCI may henceforth correspond to the following: (1.) Cognitive complaint emanating from the patient and/or his/her family; (2.) the subject and/or notifying party report a decline in cognitive and/or functional performance relative to previous abilities; (3.) cognitive disorders evidenced by clinical evaluation: impairment in memory and/or another cognitive domain; (4.) cognitive impairment does not have any repercussions on daily life, the subject may report difficulties concerning complex day-to-day activities; (5.) no dementia.