The advent of missile seekers with dual-mode millimeter wave and infrared common-aperture sensors has led to a requirement to develop the simulation tools necessary to test these systems. Traditionally, one of the most important techniques for supporting systems development has been a full seeker hardware-in-the-loop simulation. Over the past twenty years, there has been a substantial effort to develop the required simulators for supporting development of systems that utilize single-mode guidance. However, the development of comparable simulation facilities capable of supporting the new generation of advanced dual- mode guided systems has been limited due to some major technological challenges that are not readily overcome. This paper provides an overview of the simulation facilities available at the U.S. Army Missile Command to support systems development of single-mode and non- common-aperture dual-mode systems, presents the major technological challenges that limit common-aperture dual-mode simulator development, and provides a conceptual description of a dual-mode simulator capable of providing simultaneous imagery to both sensors of a common-aperture seeker.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.