This discipline explains how to provide feedback about the maturing system by designing, implementing, running, and evaluating tests. 

The purpose of this discipline is to:

• Provide early and frequent feedback that the system satisfies the requirements
• Objectively measure progress in small increments
• Identify issues with the solution
• Provide assurance that changes to the system do not introduce new defects
• Improve velocity by facilitating the discovery of issues with requirements, designs, and implementations as early as possible

The Test discipline is iterative and incremental. It applies the strategy of "test early and test often" in order to retire risks as early in the system’s lifecycle as possible.

Testing occurs in each iteration of the lifecycle, beginning with the earliest builds of the system. In fact, it’s common for one iteration to have many test cycles, depending on the frequency of new builds.

Testing asks the question: "What does the Solution have to do in order for us to consider a requirement implemented?" Tests elaborate on the requirements with specific conditions of satisfaction that the Solution must meet.

This discipline challenges the assumptions, risks, and uncertainty inherent in the development of highly technical artifacts, and addresses those concerns using concrete demonstration and impartial evaluation.