Many serious accidents have happened in the world where systems have been large-scale and complex, and have caused heavy damage and a social sense of instability. Furthermore, advanced nations have almost ?nished public inf- structureandrushedintoamaintenanceperiod.Maintenancewillbemore- portant than production, manufacture, and construction, that is, more ma- tenance for environmental considerations and for the protection of natural resources. From now on, the importance of maintenance will increase more and more. In the past four decades, valuable contributions to maintenance policies in reliability theory have been made. This book is intended to s- marize the research results studied mainly by the author in the past three decades. The book deals primarily with standard to advanced problems of main- nance policies for system reliability models. System reliability can be mainly improved by repair and preventive maintenance, and replacement, and rel- bility properties can be investigated by using stochastic process techniques. The optimum maintenance policies for systems that minimize or maximize appropriate objective functions under suitable conditions are discussed both analytically and practically. The book is composed of nine chapters. Chapter 1 is devoted to an int- duction to reliability theory, and brie?y reviews stochastic processes needed for reliability and maintenance theory. Chapter 2 summarizes the results of repair maintenance, which is the most basic maintenance in reliability. The repair maintenance of systems such as the one-unit system and multiple-unit redundant systems is treated. Chapters 3 through 5 summarize the results of three typical maintenance policies of age, periodic, and block replacements.