In this paper, we give an overview of the diagnostic categories of autism and other pervasive developmental disorders (PDDs) and discuss the changes in the DSM classification system over the past 20 years. We describe each subtype of PDD, along with comorbid psychiatric conditions, assessment guidelines, and tools for diagnosis. The epidemiology of autism has generated much discussion and research; we report the most recent data, as well as recent findings about controversial issues purporting to cause the increased prevalence rate observed in the past decade. Finally, we discuss the prognosis for individuals with autism, indicating the challenges faced by patients, families, and professionals aiming to optimize their outcome.