Extreme Programming (or XP) is a software engineering methodology (and a form of agile software development) prescribing a set of daily stakeholder practices that embody and encourage particular XP values (below). Proponents believe that exercising these practices-traditional software engineering practices taken to so-called "extreme" levels-leads to a development process that is more responsive to customer needs ("agile") than traditional methods, while creating software of better quality.

Proponents of Extreme Programming and agile methodologies in general regard ongoing changes to requirements as a natural, inescapable and desirable aspect of software development projects; they believe that adaptability to changing requirements at any point during the project life is a more realistic and better approach than attempting to define all requirements at the beginning of a project and then expending effort to control changes to the requirements.

However, XP has been noted for several potential drawbacks, as compared to more document-based methodologies, including problems with unstable requirements, no documented compromises of user conflicts, and lack of an overall design spec or document (see below: Controversial aspects).

Goals of XP

"Extreme Programming Explained" describes Extreme Programming as being:

• An attempt to reconcile humanity and productivity
• A mechanism for social change
• A path to improvement
• A style of development
• A software development discipline

The main aim of XP is to reduce the cost of change. In traditional system development methods (such as SSADM) the requirements for the system are determined at the beginning of the development project and often fixed from that point on. This means that the cost of changing the requirements at a later stage (a common feature of software engineering projects) will be high.

XP sets out to reduce the cost of change by introducing basic values, principles and practices. By applying XP, a system development project should be more flexible with respect to changes.

Principles

The principles that form the basis of XP are based on the values just described and are intended to foster decisions in a system development project. The principles are intended to be more concrete than the values and more easily translated to guidance in a practical situation.

Feed back

Extreme Programming sees feedback as most useful if it is done rapidly and expresses that the time between an action and its feedback is critical to learning and making changes. Unlike traditional system development methods, contact with the customer occurs in more frequent iterations. The customer has clear insight into the system that is being developed. He or she can give feedback and steer the development as needed.

Unit tests also contribute to the rapid feedback principle. When writing code, the unit test provides direct feedback as to how the system reacts to the changes one has made. If, for instance, the changes affect a part of the system that is not in the scope of the programmer who made them, that programmer will not notice the flaw. There is a large chance that this bug will appear when the system is in production.

Assuming simplicity

This is about treating every problem as if its solution were "extremely simple". Traditional system development methods say to plan for the future and to code for reusability. Extreme programming rejects these ideas.

The advocates of Extreme Programming say that making big changes all at once does not work. Extreme Programming applies incremental changes: for example, a system might have small releases every three weeks. By making many little steps the customer has more control over the development process and the system that is being developed.

Embracing change

The principle of embracing change is about not working against changes but embracing them. For instance, if at one of the iterative meetings it appears that the customer's requirements have changed dramatically, programmers are to embrace this and plan the new requirements for the next iteration.