X Driven Development, is what we use to determine if the software we are building works in accordance with what we expect it to do. But how do we know what we are building is in accordance with the actual business domain? Do you look to the 500 page requirements document for the answers? Do you look at the wire frame that looks nothing like what you have actually designed? Do you trust the acceptance test that the product owner has authored and believes this is the way it should work? What do you do?