Intellectual disability (ID) is a disabling condition associated with deficits in intellectual functioning and adaptive behavior. Those involved in the assessment of children and adolescents with ID should consider recent changes in diagnostic criteria for the condition, as well as best practices in assessment. The American Association on Intellectual and Developmental Disabilities provides a multidimensional framework of human functioning encompassing five domains: intellectual abilities, adaptive behavior, health, participation, and context of the individual. Those engaged in assessment of ID should be aware of psychometric considerations when they are selecting and completing cognitive ability test batteries. The chapter reviews some controversies associated with ID assessment and offers informed opinions to guide the application of research to practice. The accurate assessment of ID is of utmost importance, and thus clinicians should act purposefully to ensure that their test results are valid. When a clinician is considering ID as a diagnosis, a number of factors may confound the assessment process. These factors fit into two broad categories: competing hypotheses regarding the appropriate diagnosis, and construct-irrelevant influences that reduce the validity of the assessment results. In addition to considering competing hypotheses and construct-irrelevant influences, clinicians should consider the influence of culture and language. (PsycINFO Database Record (c) 2019 APA, all rights reserved)