Before the diagnostic manuals the DSM-5 (2013) and ICD-11 (2022) were adopted, what is now called ASD was found under the diagnostic category pervasive developmental disorder. The previous system relied on a set of closely related and overlapping diagnoses such as Asperger syndrome, and Kanner syndrome. This created unclear boundaries between the different terms, so for the DSM-5 and ICD-11, a spectrum approach was instead taken. The new system is also more restrictive, meaning fewer people qualify for diagnosis after the change.

The DSM-5 and ICD-11 use different categorisation tools to define this spectrum. DSM-5 uses a “level” system, which ranks how in need of support the patient is. Meanwhile, the ICD-11 system has two axes: intellectual impairment and language impairment, as these are seen as the most crucial factors.

It is now known that autism is a highly variable neurodevelopmental disorder which is generally thought to cover a broad and deep spectrum, manifesting very differently from one individual to another. Some have high support needs, may be non-speaking, and experience developmental delays; this is more likely with other co-existing diagnoses. Other individuals have relatively low support needs; they may have more typical speech-language and intellectual skills but atypical social/conversation skills, narrowly focused interests, and wordy, pedantic communication. They may still require significant support in some areas of their lives. The spectrum model should not be understood as a continuum running from mild to severe, but instead means that autism can present very differently in each individual. How a person presents can depend on context, and may vary over time.

While the DSM and ICD are greatly influenced by the other, there are also differences. For example Rett syndrome was included in ASD in the DSM-5 but in the ICD-11 it was excluded and placed in the chapter for Developmental Anomalies. The ICD and the DSM change over time, and there has been collaborative work towards a convergence of the two since 1980 (when DSM-III was published and ICD-9 was current), including more rigorous biological assessment - in place of historical experience - and a simplification of the system of classification.