This chapter motivates the need for new agile model-based testing mechanisms that can keep pace with agile software development/evolution. A new concentric loop-based technique, which effectively utilizes resources during iterative development, is presented. The tightest loop is called crash testing, which operates on each code check-in of the software. The second loop is called smoke testing, which operates on each day’s build. The third and outermost loop is called the “comprehensive testing” loop, which is executed after a major version of the software is available. Because rapid testing of software with a graphical-user interface (GUI) front-end is extremely complex, and GUI development lends itself well to agile processes, the GUI domain is used throughout the chapter as an example. The GUI model used to realize the concentric-loop technique is described in detail.