Over the past century, our understanding of autism spectrum disorder (ASD) has drastically deepened. While originally classified as a subcategory of schizophrenia (Macoun et al., 2023, Autism across the ages: An abbreviated history. In J. L. Matson & P. Sturmey (Eds.), Handbook of autism and pervasive developmental disorder. Autism and child psychopathology series. Springer), ASD is now recognized as an independent neurobehavioral developmental disorder impacting an individual’s behavioral, social, communication, and cognitive needs (APA, 2022, Diagnostic and statistical manual of mental disorders (5th ed., text rev.). American Psychiatric Association). Additionally, unlike homogenous development disorders that can be traced to a single root cause, such as Down syndrome, ASD is considered to be heterogeneous as the root causes of autism and how it manifests across individuals can vary (Masi et al., 2017, Neuroscience Bulletin, 33(2), 183–193). This chapter will explore the influences related to the heterogeneous of autism through a discussion of diagnostic criteria history, influencing factors such as genetics and environment, variations in cognitive functioning and symptoms variability, and current evidence on treatment and supports for within autism.