Agree, but I'm concerned all these rules of thumb are doing nothing to help the authors of bad test suites understand why they're bad and how to fix them. In m

Agree, but I'm concerned all these rules of thumb are doing nothing to help the authors of bad test suites understand why they're bad and how to fix them. In my opinion the point of building out a test suite isn't to reach 100% code coverage, it's to allow developers working on the application the ability to refactor quickly and aggressively during the development process while remaining confident that it will still perform correctly when they're ready to release. Poorly written tests (or over-testing) will slow down refactoring and hinder development. But, in my experience, not having the right tests can mean firing up your application and manually testing after each small code change to prevent regression. Understanding the trade-off is key to helping developers answer the ubiquitous question: "do I need to test this"? This role for unit tests is described effectively in Chapter 1 of Martin Fowler's awesome Refactoring book (or Refactoring, Ruby Edition, if that suites you. That's the version that's on my desk right now.) This use case is also highlighted in Eric Evans' book Domain Driven Design, where he highlights one of my favourite application development techniques: "refactoring to deeper insight." Both books are among my favourites. Adding features to an application isn't just about appending files and lines of code, it's also about changing existing code to allow the new features to fit comfortably into the application's domain model. If you can accomplish that goal with "light integration tests" go for it. That'll mean much less overhead during refactoring and will do a good job of enforcing correctness. If your application contains a robust domain model (which it very well might) you may find unit tests useful for maintaining model integrity during development. This is probably what's meant by "Naturally, some parts of a complex app will behave like a library". When you change the domain model you'll have to change tests for all the affected classes and that's a good thing. It explicitly highlights how your change affected the model, which you should certainly understand before committing your changes. I will admit this distinction is hard to communicate to first-time TDDers. However, I've found that after having to eat their own dog food for a few iterations (adding features to your own code) smart developers quickly find the sweet spot. The less code you write, the less you have to maintain. But too few test or poorly written tests can slow you down just as much.