Mild cognitive impairment (MCI) was previously defined as a transitional state that can precede dementia, but the condition and the rates of conversion remain controversial. MCI is now the focus of natural history studies, along with Alzheimer's disease (AD) prevention. The objective of our review will be to consider the question of whether MCI is a well enough established entity that it can be a diagnosis in medical practice and a valid target of Alzheimer's prevention therapy. MCI was originally defined by Petersen et al. (1999) as progressive memory loss, prodrome of Alzheimer's disease. More recently MCI has been expanded to other cognitive domains with other potential causes like normal aging, fronto-temporal dementia, and vascular dementia. Despite many consensus conferences, experts cannot agree on critical aspects of the MCI, particularly with respect to its clinical utility. Based on neuropsychological studies, a hippocampal memory profile has been proposed for MCI as prodromal AD. Further research is needed to advance these criteria. We have no doubt, however, that in the future, the diagnosis of AD as disease (not only a dementia syndrome) will be made in the early pre-dementia stage and will be drawn from a combination of neuropsychological, neuro-imaging and CSF biomarkers.