Test Driven Development (TDD) is arguably the most impactful Agile practice. Nobody even talks about it anymore, but automated tests were revolutionary when they came on the scene. Fifteen years ago, the common practice was to have a big monolithic application. The developers would produce a build (usually an executable) once in a blue moon, throw it over the fence to the QA department which would bang on it for a while and then open tens if not hundreds of bugs. Then, the developers would engage is a bug squashing period.
Luckily, that's no longer the case and everyone recognizes the importance of automated tests and rapid iterations. TDD means that your whole development process is driven by tests. When you think about problems you phrase the discussion in terms of what tests you'll need. When you model things you consider how testable they are. You might change choose between alternatives based on the impact on your test suite.
The ultimate in TDD is the test first movement. This is truly putting tests on a pedestal and treating tests as the most crucial building blocks of your system and its architecture. There are multiple advantages to TDD. The best one is that you get to say TDD all the time. Try it. It's fun: TDD, TDD, TDD. Almost as important is the fact that you will have automated tests for all your code and those tests will even be kept up to date as you evolve and refactor your system. Don't underestimate TDD. Every development team starts with the best intentions regarding all kinds of best practices, but as the pressure mounts many of them go down the drain: test coverage, modularity, coding conventions, refactoring, clean dependencies, etc.
Having tests as your primary driver ensures they will not be neglected, which will give you better a chance at evolving your systems.
TDD, Test Driven Development, automated testing, Software Development Lifecycle