The reliability at timetof a system in sustained operation is often taken to be the probability that it functions continuously during the time interval [0,t]. The standard computation of system reliability finds the probability that the system functions at timetin terms of the probabilities that its components function at timet. This procedure is relevant only if the system, and its components, havelives(roughly speaking, a device has a life if it functions continuously until some time of failure, and remains failed thereafter).