In 2002, the United States Supreme Court held in the landmark case of Atkins v. Virginia (2002) that the execution of individuals who have mental retardation is unconstitutional. Following the Atkins holding, courts in death penalty jurisdictions have relied heavily upon mental health professionals in making a determination of whether or not capital offenders have mental retardation. The determination of mental retardation in death penalty cases, however, presents complex challenges for both courts and mental health professionals. In addition, there is variability in how death penalty states define mental retardation and in the assessment methods used by mental health professionals to diagnose mental retardation in such cases. The purpose of this article is to (a) describe how statutes in death penalty jurisdictions have operationalized the various clinical definitions of mental retardation, (b) discuss issues confronting examiners in assessing and diagnosing mental retardation in Atkins cases, and (c) provide recommendations for forensic practice.