A description of autism spectrum disorder (ASD) as defined by DSM-5 is provided such that the key areas to be covered in the assessment of ASD are evident. As discussed in this chapter, ASD, separate from and co-occurring with, intellectual disability is a critical determination that can be made during the assessment. The discussion includes the implications of the heterogeneity of ASD for the assessment process. Specific instruments that are commonly used in these assessments, including their format, strengths, and limitations are presented.