School psychologists, social workers, speech and language pathologists, and special education teachers are increasingly tasked with identifying children with autism spectrum disorders (ASD). This chapter defines the core social communication and restricted repetitive behaviors present in ASD. Over the last century, psychiatric and educational definitions of ASD have evolved. At the same time, prevalence rates of ASD have increased exponentially in both the medical and educational settings. Finally, the trajectory of ASD is explored, including early ASD symptoms, developmental regression, language development, optimal outcomes, and postsecondary educational and work outcomes.