MF Bliki: CodeAsDocumentation
This principle comes with a important consequence - that it's important that programmers put in the effort to make sure that this code is clear and readable.
I’m quite a fan of XP and agile in general. There’s a common misconception about documentation as related to XP, meaning that XP prescribes not to write any documentation. In a way it’s almost true, but it was just to question the value of all the documents that some process require. When you really think about it, an awful lot of them won’t make the final product any better, nobody will really want to read them, and they will probably be out of date.
Which is why I really like this quote from Martin Fowler. It added one more element to why I think why the XP practices, when well understood, lead to a good program, with good code and good design. I strongly suggest you read the whole article.