In general terms, a system is a combination of items that are interconnected with each other to perform a specific operational function or functions. At its highest level, a system may consist of a number of individual pieces of equipment, each designed to perform a particular function as a self-contained unit; alternatively, at the lowest level of assembly, a system may be a combination of individual electronic components and/or mechanical parts providing an input function to the next higher level of assembly. Clearly, any combination of items between these two extremes may also form a system. Therefore, it is essential to define clearly the boundaries of the system under consideration. (This is described more fully in Reliability Prediction Methods.)

Providing that a system is capable of performing its functions at some point in time, it will continue to have that capability until the operating characteristics of a component or part (or group of components/parts) changes to the extent that the specified function of the system is no longer achieved. The reliability of a system, therefore, depends upon:

To predict system reliability, the relationships between these factors must be established. Such relationships are described in the following section, Reliability Block Diagrams.