This paper examines the most widely used reliability models. The models discussed fall into two categories, the data domain and the time domain. Besides tracing the historical development of the various models their advantages and disadvantages are analyzed. This includes models based on discrete as weil as continuous probability distributions. How well a given model performs its purpose in a specific economic environment will determine the usefulness of the model. Each of the models is examined with actual data as to the applicability of the error fmding process.