This chapter presents some basic reliability calculations along with step-by-step illustrations of their application to various situations. Reliability calculations can be somewhat baffling to those who are being introduced to them. Mean time between failures calculations(MTBF) may be made from test data or from field observations. To estimate the MTBF for reliability tests in which the failure rate is considered constant, divide the total number of accumulated device hours by the total number of relevant failures. Many reliability tests are time censored; that is, they are terminated after a certain time, regardless of how many of the items under test have failed. In evaluating data from reliability tests or in-service performance, it is apparent that, even though an item has not failed, it contributes information about the reliability of the product. Such items are called suspended items or suspensions. It is therefore desirable to have analytical methods to account for data from both failed and suspended items.