Autism spectrum disorders are diagnosed by the presence of behavioral characteristics that include significant impairments (i.e., deficit or excess) in language and communication, social interaction, and the presence of repetitive behaviors and/or interests. These impairments typically have an effect on the person's ability to communicate, form relationships with others, and respond appropriately to the environment.

Autism spectrum disorders cover a wide range of behaviors and abilities. Children diagnosed with autism (like all children) differ in their behavioral characteristics and skill levels.

Autism spectrum disorders are diagnosed by the presence of behavioral impairment in the following three areas, (a) Social skills and interaction, (b) Speech, language and communication, and (c) Repetitive patterns of behavior, interests, and activities. These characteristics will differ between children diagnosed with an autism spectrum disorder. Behavioral characteristics of autism spectrum disorders are typically identified before the age of three and are four times more likely to occur with boys than girls.

Due to the differences in impairment, each individual behaves differently. Thus, autism spectrum disorders can be difficult to diagnosis. The diagnosis of an autism spectrum disorder is typically made through assessments given by several professionals from a variety of disciplines including a psychologist, a developmental pediatrician, a speech/language pathologist, an occupational therapist, and a physical therapist. This thorough approach ensures that each of the three areas (communication, social interaction and repetitive behaviors) is properly assessed.