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The U.S. Nuclear Regulatory Commission has undertaken a program to lay the groundwork for defining proactive actions to manage degradation of materials in light water reactors (LWRs). This proactive management of materials degradation (PMMD) program examines LWR component materials and the degradation phenomena that affect them. Of particular interest is how such phenomena can be monitored and data can be used to predict degradation and prevent component failure. Some forms of degradation, including some modes of stress corrosion cracking, are characterized by a long initiation time followed by a rapid growth phase, and monitoring such long-term degradation will require new nondestructive evaluation methods and measurement procedures. As reactor lifetimes are extended, degradation mechanisms previously considered too long-term to be of consequence (such as concrete and wiring insulation degradation) may become more important. This paper explains the basic principles of PMMD and its relationship to in-service inspection, condition-based maintenance, and advanced diagnostics and prognostics. It then reviews the phases for degradation development and technologies with potential for sensing and monitoring degradation in its early stages. [ABSTRACT FROM AUTHOR]