Univariate probability models are involved in the analysis of failure times (or survival times) in two distinct domains of application. We may be dealing with a single component or with a multi-component assembly or system. And, in the second case, we usually start with a multivariate probability distribution to model the joint behaviour of component failure times and then work out the univariate distribution of the life of some system made up of these components according to some configuration. The configuration derives system life as a function of component lives. Thus, a large part of reliability and survival analysis is based on univariate probability models with relevant properties.