This paper presents selected results of four experiments into air traffic control (ATC) aspects of free flight (FF). The first two examined basic human performance implications of FF, in terms of workload and ability to monitor traffic. The third explored the potential for improved ATC displays to benefit controllers under FF traffic patterns. The fourth experiment examined methods for accommodating mixed equipage, such as during a transitional FF era in which both FF capable and FF incapable aircraft would be expected to share the same airspace. The first three experiments involved controllers operating in “open-loop” simulations, with computer-generated traffic and simulated pilot responses. In the final experiment, pilots and controllers were linked in real-time sessions. Results of the four studies can be summarised as follows: (1) Controller workload seemed more sensitive to traffic load than to whether traffic was operating under FF; (2) Difficulties in monitoring FF traffic were observed, and might indicate a fundamental difficulty in transitioning controllers into a FF scenario; (3) Improved ATC displays might improve controllers’ ability to monitor FF traffic; and (4) the way in which airspace is structured can influence controller workload, and in turn the near-term transition to FF.