With the current rise in the diagnoses and prevalence of the various forms of autism, there has been greater interest in knowing what constitutes autism within the medical world and how it is classified.

Autism is one of the pervasive developmental disorders as detailed in the DSM-IV, the primary diagnostic criteria for disorders. Pervasive developmental disorders are disorders with marked impairment in social skills, including problems with social interaction, difficulty with average communication, and the lack of expected interests and activities. On their own, each of these issues are not too much trouble, but if together all at once, they may signify the presence of autism or one of the other pervasive developmental disorders. Some of the other pervasive developmental disorders are frequently referred to as autistic disorders, which can cause some confusion outside the medical field, but the medical diagnosis of classic autism itself is different than other pervasive developmental disorders, although Asperger's Syndrome sometimes overlaps between the other pervasive developmental disorders and classic autism.

Classic autism presents in early childhood as a developmental defect in the brain, which is why it is sometimes called childhood autism, or sometimes infantile autism, and usually shows signs sometime around 15 or 30 months old. Symptoms range across the board with various levels of severity, sometimes called low-, medium-, or high-functioning autism after IQ tests determine where they are placed. Some people do not speak at all or may do nothing but rock other repetitive actions. Others can live fairly normal lives but still appear eccentric, single-minded, or have awkward social habits such as a tendency to speak in verbose, pedantic sentences. People with Asperger's Syndrome are often high-functioning. In other cases, the type of autism is classified due to whether it has a physical or congenital cause or whether it is non-syndromal. Sometimes a diagnosis of autism is rendered when a young child loses language or social skills rather than has trouble developing them in the first place, which is the typical diagnostic clue that autism is present. In some cases, mental issues in the parents are thought to be the cause of some forms of autism.

At best, these classifications are sketchy and subjective and include such a broad range of symptoms and possibilities - it is hard to determine exact standards for the classifications of the various levels of functionality, and the methods of doing so are somewhat controversial. It also makes research into a treatment or cure difficult, since there are so many different levels of autism and the origin is unclear - whether there are environmental factors that determine whether autism develops, whether autism is genetic, or whether it is a congenital defect. The fact that autism could be a variation of all three depending on the person diagnosed with autism just shows how broad and unwieldy a diagnosis of autism can be. The fuzziness of the classification of autism makes the diagnosis sometimes difficult for parents, doctors, and people diagnosed with autism to accept as unadulterated fact.