In a software development project, errors can be introduced at any stage during development. Though errors are detected after each phase by techniques like inspections, some errors remain undetected. Ultimately, these remaining errors will be reflected in the code. Hence, the final code is likely to have some requirements errors and design errors, in addition to errors introduced during the coding activity. Testing is the activity where the errors remaining from all the previous phases must be detected. Hence, testing performs a very critical role for ensuring quality. The focus of this chapter is primarily on system testing in which the entire software system is tested, though testing is also performed on individual programs written by programmers and the concepts discussed are also applicable for individual program testing.

During testing, the software to be tested is executed with a set of test cases, and the behavior of the system for the test cases is evaluated to determine if the system is performing as expected. Clearly, the success of testing in revealing errors depends critically on the test cases. Much of this chapter is devoted to test case selection, criteria for selecting test cases, and their effect on testing.

We begin this chapter by discussing some definitions and concepts pertinent to testing. Then we discuss the two basic approaches to testing black box or functional testing and white-box or structural testing. Aspects of testing process is discussed next, followed by a discussion on how testing data can be used for defect prevention.