Systems, predominantly computing systems, tele­communication systems, and other computer-based systems, which tolerate undesired changes in their internal structure or external environment. Such changes, generally referred to as faults, may occur at various times during the evolution of a system, beginning with its specification and proceeding through its utilization. Faults that occur during specification, design, implementation, or modification are called design faults; faults that occur during utilization are referred to as operational faults. Design faults are due to mistakes made by humans or by automated design tools in the process of specifying, designing, implementing, or modifying a system. Operational faults can be physical or human and can occur internally or externally. Examples include component failures (physical-internal), temperature and radiation stress (physical-external), mistakes by human operators who are integral parts of the system (human-internal), and inadvertent or deliberate interference by humans who interact with the system (human-external).