Coupling and abstraction are closely related: by introducing a method to reify an abstraction, you often naturally decrease the coupling between two classes. We'll explore this with a simple model/controller example, as well as a case where the situation isn't so straightforward: a method call that looks like a good, simple abstraction but is actually dangerous because it's third-party. Finally, we'll see the way that isolated, outside-in TDD naturally encourages you to build good abstractions early.