The term reliability usually refers to the probability that a component or system will operate satisfactorily either at any particular instant at which it is required or for a certain length of time. Fundamental to quantifying reliability is a knowledge of how to define, assess and combine probabilities. This chapter shows how the pattern of outcomes, that is, the way in which total probability is distributed over all possible outcomes. This may be presented in the form of a set of separate numeric values, or, in the case of well-defined patterns of probability distribution, as an algebraic function. The chapter also illustrates a discrete probability distribution in the form of a histogram of theoretical probabilities, where the heights of the bars are proportional to p(x) for each discrete value of the variable x. Expected values may involve some condition, such as the factory source giving rise to the probability distribution.