Successfully rewriting (or refactoring) a legacy system is a tricky and difficult task because there are no accurate requirements; just make it do what the current system does. Even if the original requirements documents are available it is almost guaranteed that they’re out of date and only describe part of what the system does. So what’s a developer to do? Fortunately, that’s what this talk is about. You’ll learn a practical way of ensuring you know what the current system does, and that the new version (whether a rewrite or a refactoring) does the same thing. You’ll also learn how to prevent any systems you’re currently working on from having out-of-date requirements, without spending time updating the requirements document.