This paper briefly notes the use of computers in ATC and then discusses the different methods which are available for evaluating and planning airport and airspace systems giving the advantages and disadvantages of each approach. It aims to show that when dealing with a complex system Fast-Time Simulation (based on the Event-to-Event Simulator Technique) has a significant role to play in determining how a system will behave under increased traffic load or after the introduction of new facilities. By investigating system behaviour, it is possible to identify bottlenecks which create congestion and delay and to plan them out of the system thereby creating significant savings in the use of aviation fuel.