This paper describes motivation, design, current and potential future application of the Multi Aircraft Control System (MACS). MACS is a powerful research tool that is being developed at NASA Ames Research Center to increase the overall realism and flexibility of human-in-theloop air traffic simulations. MACS is designed to enable many participants to be included in the same simulation, onor off-site. Each MACS station is a platform independent Java program that provides user interfaces and views for pilots, air traffic controllers/managers, airline dispatchers, experiment managers, and observers. Any station can serve as a mid-fidelity input device, an autonomous agent or a display for any perspective of a distributed air traffic management simulation. MACS is laid out for rapid prototyping of user interfaces, air traffic control as well as flight management and guidance functions. Domain and human-factors experts can collaborate with software engineers, quickly prototype new features and operational concepts and evaluate them from different perspectives. In this paper we present some of the currently available views, cover specific research capabilities such as repeatable situation awareness probes, describe the overall simulation layout, and give an example of MACS usage in a Distributed Air Ground simulation study that is currently being conducted at NASA Ames Research Center. Funding for this work was provided by the Advanced Air Transportation Technologies Project of NASA’s Airspace Systems Program.