Incidence and prevalence, the measures of "frequency, " are often confused. While in a nonhereditary situation, the useful parameter is the incidence rate, evaluating the impact of an etiologic factor, it is prevalence that is considered useful in a hereditary disease. Prevalence may concern either the whole population or a fraction of this population, that is, males or females or individuals at a given age, for example, at birth. Pathologic phenotype and morbid genotype prevalences have to be clearly differentiated. In this article, we review the epidemiologic surveys allowing an estimation of the distribution of major single-gene kidney diseases progressing to renal failure in different populations. In order to compare their results, the geographic/ethnic composition of the population, the determination of its size, the choice and mode of calculation of the epidemiologic measure, the definition of the disease and modes of diagnosis, the inclusion of cases, the sources of ascertainment and the possible causes of underascertainment, and the period of time during which events were counted should be analyzed accurately. Although their impact in terms of morbidity, hospitalizations, mortality, and cost to society is high, this review shows that information on the prevalence of single-gene kidney diseases is far from complete. To date, the data essentially apply to large populations of European origin. A part of the variation among prevalence data may be due to methodological differences. Not representative are the small populations in which some rare diseases, especially recessive, are found with a high prevalence.