Simulation is a vital tool in the development of communications modems, channel sounding and evaluation systems, radio control systems, etc. It enables testing under a variety of controlled conditions and avoids the expense of on-air tests. However, for meaningful exchange of performance information between cooperating development groups, the simulators they use must be validated, i.e. shown to meet some given standard. This paper discusses the validation tests used and presents two simulators, one implemented in hardware and the other in software. (6 pages)