FMS is two or more computer controlled units interconnected with automated workhandling equipment and supervised by an executive computer having random scheduling capabilities. The complex nature of a FMS, coupled with the need to maximize its performance, means that such a system can be effectly designed only by iterative computer simulation methods. At the operational level, simulation had a reduced role to evaluate the performance of planned FMS under a variety of operating strategies. This is probably due to the fact that few simulation models are able to access the current status data of the system. This study reviews the role of simulation for design/operation of FMS and suggests an interface scheme for incorporating a simulation package with a FMS operational database. This scheme can be used for reducing the performance gap between design and real operation of a FMS.