This chapter provides a thorough coverage of the parameters, and their inter-relation, used in reliability. The importance of a precise definition of failure is emphasised. Parameters are introduced (see keywords) and their relationships developed – in particular the relationship of failure rate to reliability. Approximations are given, where relevant, and their limitations explained. The bathtub relationship of failure rate with time is covered and down time is broken into elements of activity. The importance of using the relevant parameter is emphasised.