Predicting the performance of high speed wide area ATM networks (WANs) is a difficult task. Evaluating the performance of these systems by means of mathematical models is not yet feasible. As a result, the creation of simulation models is usually the only means of predicting and evaluating the performance of such systems. In this paper, we use measurements to validate simulation models of TCP/IP over high speed ATM wide area networks. Validation of simulations with measurements is not common; however, it is needed so that simulation models can be used with confidence to accurately characterize the performance of ATM WANs. In addition, the appropriate level of complexity of the simulation models needs to be determined. The results show that under appropriate conditions simulation models can accurately predict the performance of complex high speed ATM wide area networks. This work also shows that the user perceived performance is dependent on host processing demands.