Reliability theory has become one of the important areas in Operational Research and Systems Engineering. Any system analysis, in order to be complete, must give due consideration to system reliability and availability. A system deSigner is often faced with the problems of evaluation and improvement of system relia bility and determination of optimum preventive maintenance schedule. In the solution of these problems, he is largely aided by mathematical mOdels. These models have received consi derable attention in the past several years following the satel lite era. The monographs of Gnedenko,...