Historically, the development of advanced automation for air traffic control in the United States has excluded the input of the air traffic controller until the end of the development process. In contrast, the development of the Final Approach Spacing Tool (FAST), for the terminal area controller, has incorporated the end-user in early, iterative testing. This paper describes controller involvement in three successive phases of system development, tailored to the complexity of the software testing process. In addition, the creation of a Controller Acceptance Rating Scale to aid user assessment and guide system design is described. This approach has enhanced the usability of FAST, and its introduction into the operational environment.