Considerable progress in understanding the nature of autism and related disorders has been made over the past 51 years. Given the early onset of the condition, it is somewhat paradoxic that our knowledge of autism in infants and very young children remains limited in important respects. Our knowledge of the other pervasive developmental disorders is even more limited. Although it now appears that these conditions arise as a result of some insult to the developing central nervous system, precise and testable pathophysiologic mechanisms have not yet been identified. Advances in the understanding of the psychology of autism should provide better directives for biologic research.