In the past, nuclear testing and the continuous development of new nuclear weapons have been the basis for confidence in the safety, reliability, and performance of the nuclear weapons stockpile. However, due to recent changes in the U.S. nuclear posture, confidence in the stockpile must now be sustained without these activities. Specifically, the Department of Energy (Department) has ceased conducting nuclear tests, production of fissile materials, and production of new-design nuclear warheads. The Department must now rely on scientific understanding and expert judgment, rather than on nuclear testing and the development of new weapons, to predict, identify, and correct problems affecting the stockpile. The scientific understanding and judgment will be based on the knowledge created by the Department and its predecessor agencies throughout the history of the nuclear weapons program, as well as such factors as nonnuclear testing and advanced computer modeling.