In this chapter, we describe a device in terms that relate to its functional lifetime. As defined by the IEEE, dependability, or reliability, describes the ability of a device to function under stated conditions for a specified period of time.1 That specified period of time is the lifespan of the device. Such a concept applies clearly to devices that fail precipitously, such as a light bulb that burns out, but the stated conditions are meant to be broader than that. They may include performance characteristics, such as capacity or efficiency, so the concept of dependability also applies to devices that lose some of their functionality over time or with use, rather than precipitously. In the latter case, it is ...