Sam Gentile talks about the value of FIT as a conversation between developers and customers to elicit business requirements. One of the things I found early on in XP was the importance of the customer defining acceptance tests. In waterfall methodologies a lot of effort goes into the requirements document, which is then used to produce acceptance tests. XP drops the requirements document in favour of a story – the promise of a conversation – but still requires the acceptance document, just accepting it may come late into the iteration. You still need the acceptance document, without it you will not be able to determine when the software you are building it is finished. The tendency to drop it and rely only on the story is a real source of problems in XP. Focus on getting it delivered. FIT is a good way to get the acceptance test in a form that can drive tests, but does not change this necessity.