run on the program to verify that the program runs as expected. Test automation, mostly using unit testing, is a key feature of extreme programming and agile software development, where it is known as test-driven development (TDD) or test-first development. Unit tests can be written to define the functionality before the code is written. However, these unit tests evolve and are extended as coding progresses, issues are discovered and the code is subjected to refactoring. Only when all the tests for all the demanded features pass is the code considered complete. Proponents argue that it produces software that is both more reliable and less costly than code that is tested by manual exploration. It is considered more reliable because the code coverage is better, and because it is run constantly during development rather than once at the end of a waterfall development cycle. The developer discovers defects immediately upon making a change, when it is least expensive to fix. Finally, code refactoring is safer when unit testing is used; transforming the code into a simpler form with less code duplication, but equivalent behavior, is much less likely to introduce new defects when the refactored code is covered by unit tests. Some software testing tasks (such as extensive low-level interface regression testing) can be laborious and time-consuming to do manually. In addition, a manual approach might not always be effective in finding certain classes of defects. Test automation offers a possibility to perform these types of testing effectively. Once automated tests have been developed, they can be run quickly and repeatedly. Many times, this can be a cost-effective method for regression testing of software products that have a long maintenance life. Even minor patches over the lifetime of the application can cause existing features to break which were working at an earlier point in time. While the reusability of automated tests is valued by software development companies, this property can also be viewed as a disadvantage. It leads to the so-called "Pesticide Paradox", where repeatedly executed scripts stop detecting errors that go beyond their frameworks. In such cases, manual testing may be a better investment. This ambiguity once again leads to the conclusion that the decision on test automation should be made individually, keeping in mind project requirements and peculiarities. Test automation tools can be expensive and are usually employed in combination with manual testing. Test automation can be made cost-effective