Simulation is an important tool in the study of communication networks manufacturing systems, and other complicated man-made systems. Many systems of this type are examples of discrete- event dynamic systems (DEDS) for which analytical models have not yet been developed; thus simulation is the primary method for their performance evaluation and control. Although in many cases simulation can provide a good estimate of system performance, it can also be extremely time consuming, and therefore expensive. This is especially true when it is necessary to evaluate system performance for a large number of values of one or more parameters or for different control policies.