Simulated images have been used for training for at least thirty years. This has included simulation of visual scenes and of displays from various sensor systems such as radar, sonar, infrared, and television cameras. Techniques used have included scale models, acoustic tanks, and photography. Increasingly through the years, analog and digital computation equipment has been used along with these techniques for control and modification of results. In the early sixties, the increasing power of digital computation and data processing circuitry led to consideration of systems in which the "environment" hills, targets, buildings, etc. exists only as stored numbers, and images are generated in real time by appropriate computations performed on these numbers. Such approaches promised many advantages as compared with traditional techniques. Continuous effort has been applied since that time to develop algorithms, overcome disadvantages, improve performance, and reduce cost. The results: extremely realistic systems are now in operation. Recent radar simulation systems are based on computed displays. Visual scene simulation is proving itself on several operating systems. These have been combined in correlated multi-mode displays. Development is under way to apply this technology to simulation of displays from other sensors.