Reliability is the probability that a system functions correctly, in a given environment over a given amount of time. Some aspects of reliability are availability, correctness, safety, operational usability, etc. Software reliability engineering is the study of operational profiles to determine the best areas to test by estimating the components that will be used the most by the customer and testing those components first.

There are several phases of software reliability engineering. The first is to define the product by looking at the customers, users, external suppliers, and certifying external software. The second step is to implement the operational profiles, which involves looking at system components and determining the probability of their use. Third, the failure intensity object is determined. Fourth, the test cases are created that fit the operation profile and is within the bounds of the failure intensity object. Lastly, reliability data is tracked so that future estimates will be more accurate.