Summary: Probability models and statistical methods are a popular technique for evaluating the reliability of computer software. This paper reviews the literature concerning these methods, with an emphasis on the historical perspective. The use of stochastic techniques is justified, and the various probability models that have been proposed, along with any associated statistical estimation and inference procedure, are described. Examples of the models applied to real software failure data are given. A classic software development problem -- how long software should be tested before it is released into the marketplace -- is analyzed from a decision theoretic standpoint. Finally, the direction of future research is contemplated.