OPINION STATEMENT: The assessment of autism spectrum disorder (ASD) is complex and remains clinical, despite advances in basic research. In this chapter, we review new and updated clinical tools, such as screening and diagnostic tests, and discuss the DSM-5 criteria introduced in 2013. We provide an algorithm to guide clinical evaluation and referrals. We also review non-behavioral treatments and summarize recent research. Current conventional treatment of ASD in children includes intensive behavioral interventions (known as applied behavioral analysis or ABA), rehabilitative services such as speech therapy, occupational therapy, physical therapy, social skills training, and counseling. We present new validated information and provide clinical guidance for the evaluation and treatment of young children and youth with ASD.