A brief of amici curiae American Association on Mental Retardation, et al., supports the petitioner in Johnny Paul Penry v. James Lynaugh, director, Texas Department of Corrections, in a writ of certiorari to the United States Court of Appeals for the Fifth Circuit. The brief argues that Penry, who is mentally retarded, should not have received a capital sentence due to the mitigating circumstance of his mental retardation, thus rendering the capital sentence unconstitutional as cruel and unusual punishment. An article discusses the status of mentally retarded criminal defendants in the American criminal justice system. After an overview of the issues in the Mental Health Standards as they relate to mentally retarded defendants, the article reviews the history of the treatment of retarded defendants in the criminal justice system. The characteristics of mentally retarded persons are described, along with the consequences of those characteristics. The discussion then addresses the extent to which mental retardation should be exculpatory of criminal responsibility. The critical importance of competence issues to mentally retarded defendants is then considered, followed by elaboration on dispositional issues, including civil commitment and sentencing. The article concludes with discussions of the role of mental retardation professionals in the criminal justice system and of specialized training in mental retardation for participants in the criminal justice system. A third paper provides guidelines for defense attorneys in representing mentally retarded defendants in capital cases.