Recent epidemiological studies of mental retardation were critically reviewed. Data pertaining to prevalence, associated disorders, and etiology were examined. The prevalence both of severe and of mild retardation is generally found to be 3 to 4 per 1,000, although estimates vary with gender, age, and method of ascertainment (e.g., data derived from total population screening yield even higher rates). A wide range of disorders were found to be associated with mental retardation. The etiology of up to 50% of cases was unknown. Of those remaining, prenatal factors were implicated about twice as often as peri- or postnatal factors. Discussion was focused on the implications of current findings for future research.