In striking down the death penalty for intellectually disabled and juvenile defendants, Atkins v. Virginia and Roper v. Simmons have been understandably heralded as important holdings under the Court’s Eighth Amendment jurisprudence that has found the death penalty “disproportional” for certain types of defendants and crimes. This Article argues, however, that the cases have a far more revolutionary reach than their conventional understanding. In both cases the Court went one step beyond its usual two-step analysis of assessing whether imposing the death penalty violated “evolving standards of decency.” This extra step looked at why even though intellectual disability and youth were powerful mitigators, juries were not able to reliably use them in their decisionmaking. The Court thus articulated expressly for the first time what this Article calls the “unreliability principle:” if too great a risk exists that constitutionally protected mitigation cannot be reliably assessed, the unreliability means that the death penalty cannot be constitutionally imposed. In recognizing the unreliability principle, the Court has called into serious question the death penalty for other offenders to whom the principle applies, such as mentally ill defendants. And, unlike with the “evolving standards” analysis, the unreliability principle does not depend on whether a national consensus exists against the practice. This Article identifies the six Atkins-Roper factors that bring the unreliability principle into play and shows why they make application of the death penalty to mentally ill defendants unconstitutional. The principle, which finds its constitutional home in the cases of Woodson v. North Carolina and Lockett v. Ohio, has profound implications for the death penalty, and if taken to its logical endpoint calls into question the Court’s core premise since Furman v. Georgia, that by providing individualized consideration of a defendant and his crime, the death penalty decision will be free of arbitrariness.