There are essentially three main prongs to the definition and diagnosis of the condition known as mental retardation: deficits in intellectual functioning, deficits in adaptive behavior, and onset of these deficits during the developmental period. The U.S. Supreme Court ruled in 2002 in a decision known as Atkins v. Virginia that it was essentially cruel and unusual punishment to execute a person with mental retardation, thus violating the Eighth Amendment of the American Constitution. For the purpose of this article, we focused on the issues as they relate to the second prong of the definition of mental retardation, that is, adaptive behavior. We present and discuss the primary concerns and issues related to the assessment of adaptive behavior when making a diagnosis of mental retardation in an Atkins claim case. Issues related to standardized assessment instruments, self-report, selection of respondents, use of collateral information, malingering, and clinical judgment are discussed.