Strive to test on three levels: 1 - Unit tests, low-level method testing w/ mocked or no external dependencies (such as db) 2 - Integration tests, test agains

Strive to test on three levels: 1 - Unit tests, low-level method testing w/ mocked or no external dependencies (such as db) 2 - Integration tests, test against external dependencies, such as db, apis 3 - System tests, front-end testing that covers all parts of the system using a tool such as Selenium Terminology varies and differentiation of levels, as well, but this can serve as a good guideline. You'll need to have everything automated and integrated into a build system where you can reliably kick off a build and see the results. A note on Selenium: any non-trivial setup is going to require a fair number of resources, both in time and environments. Also, you will have to dedicate time to maintaining tests and your UI changes. However, the pay-off is quite large when you can run and end-to-end set of tests on your application and know that you didn't break something. edit: formatting