The authors have examined the experimental and analytic bases for understanding the performance of each of the weapon types that are currently planned to remain in the US enduring nuclear stockpile. They have also examined whether continued underground tests at various nuclear yield thresholds would add significantly to the confidence in this stockpile in the years ahead. The starting point for this examination was a detailed review of past experience in developing and testing modern nuclear weapons, their certification and recertification processes, their performance margins, and evidence of aging or other trends over time for each weapon type in the enduring stockpile. The findings, as summarized in Conclusions 1 through 6, are consistent with US agreement to enter into a Comprehensive Test Ban Treaty (CTBT) of unending duration, that includes a standard ``supreme national interest`` clause. Recognizing that the challenge of maintaining an effective nuclear stockpile for an indefinite period without benefit of underground tests is an important and also a new one, the US should affirm its readiness to invoke the supreme national interest clause should the need arise as a result of unanticipated technical problems in the enduring stockpile.