System reliability considers the reliability of a system containing dependent or interdependent units, components, or subsystems. It concerns how the reliability of the whole system is related to its constituent parts. For instance, the reliability of an airplane can be assessed based on the reliability of its engines, avionics, navigation system, communication systems, and more. Similarly, the reliability of a laptop can be evaluated from that of its hard drive, screen, memory, power supply, battery, etc. The interactions among these subsystems, components, or units—referred to hereafter simply as “units” for simplicity—may not follow straightforward patterns but instead form a complex stochastic process. This chapter discusses methods for evaluating the reliability of an entire system based on the reliability of its constituent units.