Autism is a severe form of childhood psychopathology first described by Leo Kanner in 1943. While over the years there has been substantial controversy about many features of the syndrome, there is today some consensus as to the behavioral characteristics associated with the diagnosis. These include onset of the disorder in the early preschool years, severe and pervasive deficits in social behavior and attachments, deficits in speech and language, insistence for the preservation of sameness, unusual responsiveness to the sensory environment, self-stimulation, self-injurious behavior, isolated skill areas, and inappropriate affect. Another associated feature of many cases of autism is mental retardation. The present article describes these behavioral features as well as the application of the diagnosis and differentiation of autism from other disorders including primary mental retardation, childhood schizophrenia, developmental aphasia, and pervasive developmental disorder.