Extreme Programming (XP) is a lightweight design method developed by Kent Beck, Ward Cunningham, and others. After notable successes, XP has been generating huge interest, and no small amount of controversy. Much of the interest stems from XP's pragmatic approach to development. Key practices include pair programming, writing tests upfront, frequent refactoring and rebuild, continuous integration and testing. Key principles incremental and iterative development, working with the simplest solution, cutting out extraneous documentation, and collective code ownership.