The choice of maintenance policies dealt with in this book is governed by the interests of the authors. The focus is on basic models, not on the mathematically more sophisticated ones. In applications, maintenance models need to be tailored to the speci…c practical situation. Hence, the maintenance policies analyzed in what follows can only provide the general framework. For more comprehensive recent summaries, including such important aspects as warranty and optimal maintenance of multi-unit systems, see e.g. Gertsbakh [136], Pham [249],Wang and Pham [303], Nakagawa [237, 238], Sarka, Panja, and Sarka [268]. However, some of the maintenance models considered in what follows, although formally designed for simple systems, are actually applicable to the cost-optimal maintenance of complex systems.