The need for high resolution complex infrared scene simulation has risen out of the growing use of IR sensor systems for imaging, detection, tracking, and guidance applications. The performance of an IR sensor system is highly dependent on the infrared environment in which the system is operating which can involve a wide variety of complex threats, background environments, and atmospheric conditions. The development, integration, and testing of infrared sensor systems, such FLIR, IRST, or Missile Warning Systems, requires an IR scene simulation capability that can provide complex scenes, in real-time, to ensure effective evaluation of system performance. To address this need, a Real-time Infrared Scene Simulator (RISS) was developed. The RISS system generates digital infrared scenes in real-time to provide a realistic portrayal of the infrared scene radiance as viewed by an IR system under test in threat engagement scenarios. Scene radiance is calculated on a frame by frame basis accounting for the relevant contributions from the sky, sun, targets, terrain, and atmosphere as a function of the engagement geometry. The following paper will provide an overview of the RISS system and discuss the critical issues that apply to real-time infrared scene simulation.