Chapter 1 describes the diagnostic criteria as presented in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5-TR, 2022) and in the International Classification of Diseases (ICD-11, 2019). The language of the DSM and the ICD is not neurodiversity affirmative. However, because healthcare providers must follow the guidelines provided by these diagnostic tools, the language in this chapter (and this chapter only) uses the language of the medical model.

The criteria are divided into two major areas: (A) social and communication challenges, and (B) repetitive and restricted behaviors (RRBs). Social and communication challenges include (1) social/emotional reciprocity; (2) nonverbal communication; and (3) relationship management. Repetitive and restricted behaviors include (1) repetitive or idiosyncratic behavior; (2) inflexibility; (3) restricted interests that are intense or atypical; and (4) sensory differences. A person must show differences in all three of the social/communication differences, as well as two of the four repetitive/restricted behaviors in order to qualify for a diagnosis of autism. Individuals can meet criteria on both groups either currently or by history. The chapter closes with a description of the differences and similarities between the DSM-5-TR and ICD-11.