Mild cognitive impairment(MCI) is considered to be a transition state between normal cognition and dementia. The subtypes of MCI are highly heterogeneous in terms of etiology, presentation, and prognosis. Patients with the amnestic subtype of MCI are at a high risk of progression to Alzheimer disease (AD); this subtype may represent the prodromal stage of AD. Moreover, patients with MCI who are not aware of their memory deficits and in whom practice effects are not observed exhibit parietotemporal hypoperfusion on single photon emission CT, indicating that these findings are predictors of progression to AD. In this review, I have discussed the most current aspects related to the concept and clinical presentation of MCI.