Since 1966, over 80 epidemiological surveys of autism spectrum disorders (ASDs) have been conducted in more than 20 countries. In this chapter, we review existing prevalence estimates for ASDs and discuss methodological factors impacting the estimation of prevalence and the interpretation of changes in prevalence estimates over time. Possible explanations for an increase in the prevalence of ASD within and across populations are considered. Increases in ASD diagnostic rates cannot currently be attributed to a true increase in the incidence of ASD due to multiple confounding factors. It remains to be seen how changes to diagnostic criteria introduced in the DSM-5 will impact estimates of ASD prevalence going forward.