[Page 381]The process of clinically establishing a diagnosis of autism spectrum disorder (ASD) usually occurs in two steps: (1) screening for the possible presence of ASD and (2) an evaluation for a formal diagnosis. Once a diagnosis is assigned, clinicians and educators conduct assessments of important related skills such as adaptive behavior, motor skills, social communication, problem behavior, and cognitive development to develop an appropriate and comprehensive intervention plan. In this entry, the early screening process is discussed first, followed by the formal psychiatric diagnostic criteria and the major changes that have occurred in these criteria over the years.