This book aims to present a state-of-the-art survey of theories and methods of reliability, maintenance, and warranty with emphasis on multi-unit systems, and to reflect current hot topics: imperfect maintenance, economic dependence, opportunistic maintenance, quasi-renewal processes, warranty with maintenance and economic dependency, and software testing and maintenance. This book is distinct from others because it consists mainly of research work published on technical journals and conferences in recent years by us and our co-authors. Maintenance involves preventive and unplanned actions carried out to retain a system at or restore it to an acceptable operating condition. Optimal maintenance policies aim to provide optimum system reliability and safety performance at the lowest possible maintenance costs. Proper maintenance techniques have been emphasized in recent years due to increased safety and reliability requirements of systems, increased complexity, and rising costs of material and labor. For some systems, such as aircraft, submarines, and nuclear power stations, it is extremely important to avoid failure during actual operation because it is dangerous and disastrous.