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THE concepts of system reliability–generally defined as the ability of a system to meet its interface specifications-and of system availability–generally defined as the ability of a system to meet its interface specifications within a specified time limit–predate not only that of distributed computing but also that of the electronic computer itself. With the advent of the electronic computer and its ever increasing penetration of technological, social, and political developments, reliability and availability gained additional recognition as disciplines of serious intellectual challenge. Much of the development of new reliability techniques can be linked directly to the computer hardware developments of the last three decades. However, until relatively recently, the concept of software reliability did not receive significant attention and, when it did, it was rather narrowly focused on approaches to prevent failures; i.e., on software development and verification methodologies, and on languages and tools.