In test-driven development, the tests drive the development process and architecture. The software design evolves as the software changes during the development of new tests, and the architecture you end up with should be a consequence of the need to satisfy your tests.
Tests are thus the arbiter that decides the future of our software and declares that the software is doing what it is designed for. There are specific kinds of tests that are explicitly designed to tell us that the software is doing what it was requested: Acceptance and Functional tests.
So, while there are two possible approaches to TDD, top-down and bottom-up (one starting with higher-level tests first, and the other starting with unit tests first), the best way to avoid going in the wrong direction is to always keep in mind your acceptance rules, and the most effective way to do so is to write them down as tests.
But how can we write a test that depends on the whole software existing and working...
