User testing designs through prototyping helps us to safely explore risky ideas, get a feel for how real people will respond to our designs and most importantly lets us fail cheaply and privately instead of publicly and expensively. But how many users do we need? How do we identify what to test? How do we know the way we designed and ran the test is producing valid results? How do we recruit people? How do we integrate user testing into a project without adding unsustainable expense and time to a project?