You can definitely write successful applications without tests of any sort. However, as your application becomes more complex, it becomes more and more likely t

You can definitely write successful applications without tests of any sort. However, as your application becomes more complex, it becomes more and more likely that you'll find it difficult to make changes without breaking something else (especially as developers cycle through the codebase). Tests become significantly more important if you're planning on creating and supporting projects for 6+ months Now the good news is that you don't need to use TDD. Tests of any kind do make a difference, even if you have to write them after you've coded up a solution. Tests should be testing the most important parts of your application or the trickiest parts. Tests are there to give you confidence that your code does what you think it does AND to ensure that you don't accidentally cause regressions. If your application wasn't built with testing in mind, use test data or use a BDD tool like codeception. I don't find TDD to be necessary but I do find tests necessary. It along with automated deployment can increase the quality of your code by an order of magnitude.