A variety of clinically-defined predementia syndromes, with differing diagnostic criteria and nomenclature, have been proposed to describe nondisabling symptomatic cognitive deficits arising in elderly persons. Incidence and prevalence of different predementia syndromes vary as a result of different diagnostic criteria, sampling, and assessment procedures. The incidence rates of all predementia syndromes increase with age and are higher in subjects with less education; but age, educational background, and gender are not consistently related to prevalence rates. There is particular interest in "Mild Cognitive Impairment (MCI)" because this predementia syndrome is thought to be a prodromal phase of Alzheimer disease (AD). Several studies have suggested that most patients who meet MCI criteria will progress to AD, but rates of conversion to AD and dementia vary widely among studies. Furthermore, MCI definition is less consistent in population-based studies than clinical studies, in which progression to AD is also more consistent. To clarify the sources of discrepant findings in the literature, this review summarizes existing epidemiological studies of the defined clinical predementia syndromes and their progression to dementia.