Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairments in social communication and the presence of restricted and repetitive behaviors. The symptom severity and functional impact of ASD varies widely among individuals. While some individuals speak fluently and function with few supports, other individuals remain nonverbal and need high levels of care. Core impairments in social communication include deficits in social and emotional reciprocity; nonverbal communication; and developing, maintaining, and understanding relationships. Restricted and repetitive behaviors, interests, or activities may include repetitive motor movements, insistence on sameness, highly fixated interests, and hyperreactivity or hyporeactivity to sensory input (American Psychiatric Association, 2013; see also Volume 1, Chapter 9, this handbook). A diagnosis of ASD is based on a clinician's best clinical estimate. Several "gold standard" diagnostic instruments (e.g., the Autism Diagnostic Observation Schedule, Second Edition [Lord et al., 2012], the Autism Diagnostic Interview, Revised [Le Couteur et al., 2003]) have been developed that aid clinicians in determining whether an individual meets diagnostic criteria for ASD.Current prevalence rates indicate that 1 in 59 children in the United States has ASD and that it is more common in boys than girls (by a 4:1 ratio; Baio et al., 2018). While ASD can be reliably diagnosed