Starting from the agreed definition of reliability the use of probability theory enables the design engineer to predict the reliability of a complex system as a function of the reliability of its component parts. The characteristics of series and parallel grouping are given, together with some applications of hammock networks. The exponential survival law is derived, and the conditions under which it may be applied are discussed.The use of redundancy to improve reliability is introduced, and examples are given of the effect of various schemes upon the reliability of a small electronic system, with and without maintenance. Some possible forms of passive redundancy are described, including block redundancy with majority voting, and quadded logic.