The design of an EW trainer involves a decision to simulate EW functions via computer software or to incorporate actual EW hardware within the trainer and stimulate it with required signals. This paper compares the requirements and relative advantages of software simulation vs. hardware stimulation in EW trainers. Aspects discussed include, cost of hardware and software, computer load, trainer fidelity to real-world conditions, documentation and data requirements, interaction among EW units, testing requirements, and trainer modification. Both approaches have particular advantages and problems in each of these areas. In conclusion, the choice of simulation or stimulation, or mixture of both, in a given trainer should be based on careful study of particular circumstances and requirements.