This chapter deals with a variety of techniques for improving system reliability and availability. Underlying all these techniques is the basic concept of redundancy, providing alternate paths to allow the system to continue operation even when some components fail. Alternate paths can be provided by parallel components (or systems). The parallel elements can all be continuously operated, in which case all elements are powered up and the term parallel redundancy or hot standby is often used. It is also possible to provide one element that is powered up (on-line) along with additional elements that are powered down (standby), which are powered up and switched into use, either automatically or manually, when the on-line element fails. This technique is called standby redundancy or cold redundancy. These techniques have all been known for many years; however, with the advent of modern computer-controlled digital systems, a rich variety of ways to implement these approaches is available. Sometimes, system engineers use the general term redundancy management to refer to this body of techniques. In a way, the ultimate cold redundancy technique is the use of spares or repairs to renew the system. At this level of thinking, a spare and a repair are the same thing-except the repair takes longer to be effected.