There is today a wide range of pubLications avaiLabLe on the theory of reLiabiLity and the technique of ProbabiListic Safety AnaLysis (PSA). To pLace this work properLy in this context, we must recaLL a basic concept underLying both theory and technique, that of redundancy. ReLiabiLity is something which can be designed into a system, by the introduction of redundancy at appropriate points. John Von Neumann's historic paper of 1952 'ProbabiListic Logics and the Synthesis of ReLiabLe Organisms from UnreLiabLe Components" has served as inspiration for aLL subsequent work on systems reLiabiLity. This paper sings the praises of redundancy as a means of designing reLiabiLity into systems, or, to use Von Neumann's words, of minimising error. Redundancy, then, is a fundamentaL characteristic which a designer seeks to buiLd in by using appropriate structuraL characteristics of the 'modeL" or representation which he uses for his work. But any modeL is estabLished through a process of de Limination and decomposition. FirstLy, a "Universe of Discourse" is delineated; its component eLements are then separated out; and moreover in a probabiListic framework for each eLement each possibLe state is defined and assigned an appropriate possibiLity measure caLLed probability.