The prevalence of autism spectrum disorder (ASD) has increased markedly over the past three decades necessitating the screening, evaluating, and diagnosing of children with ASD as early as possible to ensure that children with ASD receive access the services and supports they need as early in development as possible. The period from 3 to 18 years of age represents an important period for detection and diagnosis of ASD. This chapter reviews a number of screening and diagnostic instruments. Also discussed are other instruments, procedures, and issues that are important in the assessment and diagnosis of ASD to ensure the likelihood of efficacious intervention result.