DSM-IV Criteria:

According to the DSM IV, in order to have a diagnosis of Autistic Spectrum Disorder, one must have the following criteria:

Qualitative impairment in social interaction, such as marked impairment in the use of multiple nonverbal behavior such as eye-to-eye gaze, facial expression, body postures, and gestures to regulate social interaction. One must fail to develop peer relationships appropriate to their developmental level.
Qualitative impairments in communication, such as a delay in, or total lack of spoken language (not accompanied by an attempt to compensate through alternative modes of communication such as gesture or mime), they must also display stereotypes and repetitive use of language or idiosyncratic language, and the child must have delays or abnormal functioning in at least one of the following areas with onset prior to age three years, social interaction, language used in social communications, or symbolic imaginative play (American Psychiatric Association, 1994).