Reliability is the probability that an engineering system will perform its intended function satisfactorily (from the viewpoint of the customer) for its intended life under specified environmental and operating conditions. Maintainability is the probability that maintenance of the system will retain the system in, or restore it to, a specified condition within a given time period. Availability is the probability that the system is operating satisfactorily at any time, and it depends on the reliability and the maintainability. Hence the study of probability theory is essential for understanding the reliability, maintainability, and availability of the system.