An age-replacement policy provides an optimal preventive replacement interval to minimize the maintenance costs. Our lives are supported by various systems. Failure of such infrastructural systems must be avoided as it causes financial and temporal damage to society. Therefore, maintenance, which ensures system reliability and recovery from failure conditions, plays an important role in ensuring the longer-term use of systems. In general, there are two types of maintenance: preventive maintenance, which is performed before a system failure, and corrective maintenance, which is performed after a failure. This chapter describes the series system model and provides three optimization problems for this model. It also describes the conditions for the existence of an optimal replacement interval. The chapter then describes the numerical examples of the proposed model and the effect of risk on the optimal replacement interval. It summarizes the main points of this study and points out its limitations.