Autism is classified as a neurodevelopmental disorder that manifests itself in markedly abnormal social interaction, communication ability, patterns of interests, and patterns of behavior.

Although the specific etiology of autism is unknown, many researchers suspect that autism results from genetically mediated vulnerabilities to environmental triggers. And while there is disagreement about the magnitude, nature, and mechanisms for such environmental factors, researchers have found at least seven major genes prevalent among individuals diagnosed as autistic. Some estimate that autism occurs in as many as one United States child in 150, however the National Institute of Mental Health gives a more conservative estimate of one in 1000. For families that already have one autistic child, the odds of a second autistic child may be as high as one in twenty. Diagnosis is based on a list of psychiatric criteria, and a series of standardized clinical tests may also be used.

Autism may not be physiologically obvious. A complete physical and neurological evaluation will typically be part of diagnosing autism. Some now speculate that autism is not a single condition but a group of several distinct conditions that manifest in similar ways.

By definition, autism must manifest delays in "social interaction, language as used in social communication, or symbolic or imaginative play," with "onset prior to age 3 years", according to the Diagnostic and Statistical Manual of Mental Disorders. The ICD-10 also says that symptoms must "manifest before the age of three years." There have been large increases in the reported incidence of autism, for reasons that are heavily debated by researchers in psychology and related fields within the scientific community.

There are cases of children with autism who have improved their social and other skills to the point where they can fully participate in mainstream education and social events, but there are lingering concerns that an absolute cure from autism is impossible with current technology. However, many autistic children and adults who are able to communicate (at least in writing) are opposed to attempts to cure their conditions, and see such conditions as part of who they are.