Agile methodologies, like most flexible yet powerful systems, rely on knowing exactly where things stand. You may not know exactly what’s coming in the future, but you know what you’ve already got, and roughly how close work in process is to done. This is one of the many reasons short iterations are a good idea. Not only do you know on a frequent basis (the end of each iteration) where you stand, but if what was done is acceptable to the stakeholders (and QA) and can really be considered done. The product backlog tells you what needs to be done in the future (based on current knowledge, which may change, and that’s OK) and the iteration backlog tells you what’s done and what needs to be done now. The burndown chart shows how things are going inside the iteration.