specific and steady-state availability and unavailability, which together form a basis for evaluating the dependability of a system. A system is taken as dependable if it satisfies all requirements of the customers with regard to various dependability performances and indices. The dependability deals with failures, repairs, preventive maintenance as well as with costs associated with investment and service interruptions or mission failures. Therefore, it is a very important attribute of system quality. The dependability evaluation is strongly based upon experience and statistical data on the behavior of a system and of its elements. Using past experience with the same or similar systems and elements, the prospective operation may be predicted and improved designs and constructions can be conceived. Hence, the dependability analysis makes it possible to learn from the past for better future solutions.