Since the early 1960's there has developed a great interest, both in industry and academia, in the subjects of maintenance and reliability of digital systems. The increased complexity of current systems makes this interest well justified. Costs associated with the necessity to maintain such systems represent a substantial and growing percentage of total system costs. In this book we consider many of the problems associated with diagnosing failures in digital systems, locating the source of such failures, and the related problem of design of ultra reliable systems - systems which do not fail...