This article reviews epidemiological studies of autism and related disorders. Study designs and sample characteristics are summarized. Currently, conservative prevalence estimates are: 13/10000 for autistic disorder, 21/10000 for pervasive developmental disorders not otherwise specified, 2.6/10000 for Asperger disorder, and 2/100000 for childhood disintegrative disorder. Newer surveys suggest that the best estimate for the prevalence of all autistic spectrum disorders is close to 0.6%. A detailed analysis of time trends in rates of pervasive developmental disorders in then provided. It is concluded that most of the increase is accounted for by changes in diagnostic concepts and criteria, and by improved identification. Whether or not there is, in addition to these factors, a true increase in the incidence of the disorder cannot be examined from available data.