Autism is a behaviorally defined, life-long static developmental disorder of the brain that is poised for neurobiological investigation. It affects at least 1 or 2 in 1000 persons and has a broad range of severity. It has multiple causes, with genetics playing a major role. According to the DSM-IV, defining features are impaired sociability, language and communication, and range of interests and activities. Mental deficiency is frequent but by no means universal. The cognitive profile is characteristic, occasionally with a superior but narrow talent. Perseveration, concreteness, affective blunting, and lack of insight into other persons' thinking may be conspicuous. The neurological basis of autism's many sensorimotor features, including stereotypies, is unknown. Attention and sleep are affected, and one third of individuals experience epilepsy by adulthood. Whether subclinical epilepsy plays a role in the developmental regression of the one third of the toddlers who lose their language skills and become autistic remains to be determined. Clinical neuroimaging and biochemical investigations are generally unremarkable. Fewer than 35 brains have been examined pathologically, none with modern techniques. The findings thus far suggest subtle prenatal neuronal maldevelopment in the cerebellum and certain limbic structures. Abnormalities in distributed networks involving serotonin and perhaps other neurotransmitters require further documentation.