The majority of the States theoretically permit the execution of those who commit a capital crime while under the age of 18. Such law contrasts sharply with international human rights norms. Deciding whether States are limited by such international norms in imposing the death penalty requires careful exploration of the conceptual and methodological gaps and weaknesses that plague the formation of customary international law, particularly in a human rights context. Domestic courts, acting well within the constraints of their appropriate roles, may invalidate State laws that purport to authorize the execution of juvenile offenders. They may act on a 'strong' theory of incorporation of the international death penalty norm into the supreme Federal common law, premised on the positive existence of international consensus and opinio juris derived from natural law. The courts may also rely on a 'weak' theory that uses the international norm to inform the meaning of the eighth amendment, which prohibits the use of cruel and unusual punishment. It would be indefensible to reject the binding nature of the norm simply because it arises from a decentralized process of customary international law formation. Customary international law is firmly embedded in American law, and human rights are a fundamental component of contemporary international law. A total of 157 footnotes are provided.