Mild cognitive impairment (MCI) is thought to be a prodromal phase of Alzheimer's disease, but in population-based studies MCI classification is less consistent than in clinical series, suggesting that MCI is a heterogeneous descriptor and the outcome at follow-up depends on which population is studied and how MCI is defined. We discussed in the present paper the issue of the heterogeneity of MCI and other predementia syndromes, both in clinical presentation and progression to dementia, supported by the findings of the Italian Longitudinal Study on Aging (ILSA) and other population-based and clinical-based studies.