Autism is one of the behaviorally defined developmental disorders of brain function. It has a variety of genetic and nongenetic etiologies, with etiology being unknown in the majority of children. Boys are more frequently affected than girls. Manifest in the preschool years, autism always affects sociability, communication, and the child's repertoire of activities and interests. Autism encompasses children with a broad range of severities and a variety of other signs of brain dysfunction. These include motor signs, notably stereotypies; abnormal responses to a variety of sensory stimuli; and disorders of affect and attention. A significant proportion of autistic children experience epileptic seizures and have abnormal EEGs. Neuroimaging, preferably magnetic resonance imaging, discloses abnormalities of brain development in a minority of autistic persons. The level of intelligence may range from profound mental deficiency to giftedness. The pattern of cognitive skills is likely to be uneven, typically with better nonverbal than verbal skills. In the preschool years, all autistic children have a developmental language disorder. Verbal expression may range from total lack of language to verbosity with echolalia; comprehension and language use are invariably impaired. While there is no specific pharmacologic agent to mitigate the fundamental disorder, children may benefit from drugs to treat specific symptoms such as attention disorder and seizures. Although autistic behaviors are the consequence of a static disorder of brain function, their character changes with maturation and appropriate intervention. Communication skills and sociability remain deficient but improve in all but the most severely affected children. Outcome is a function of both innate cognitive competence and the effectiveness of early intervention focused on the development of appropriate social skills and meaningful communication. Intelligent autistic adults may be educable, employable, and able to live independently, while more severely handicapped ones require a lifelong protected environment.