The definition of reliability given in Chapter 2 indicates that reliability is the probability of proper function over time. An implicit feature of this definition is the underlying assumption that across a population of identical devices, survival over time (or life length) is dispersed in a manner that is well modeled by probability and hence by a probability distribution function. Thus, the extension of the measures of reliability to include time involves the specification of probability distributions that constitute reasonable models for the dispersion of life length. This is the subject of this chapter.