In hardware-in-the-loop (HWIL) testing of infrared and electro-optical (IR/EO) sensor systems, stimulation of the unit under test (UUT) is achieved through either the projection of in-band scene radiance into the sensor's optical aperture or injection of the signals into the system's processing electronics. The injection of a signal representing the IR/EO scene into the electronics of the UUT requires an interface to translate the computer generated scene into the appropriate format for processing by the signal processing electronics. Likewise, the optical scene projection requires a pre-processing interface to translate the computer generated scene into the appropriate format for the optical projection system. Although the development of unique direct injection and optical projection interfaces can meet testing requirements, the effort can be costly, time consuming, and largely redundant from system to system. In order to provide the testing community with a configurable consumer-off-the- shelf (COTS) solution to address the unique emulation and interfacing problem of HWIL test facilites, Amherst Systems, under contract to the Air Force Flight Test Center (AFFTC) at Edwards Air Force Base, has developed the Universal Programmable Interface (UPI). In this paper, the UPI solution for HWIL interfacing will be reviewed. First, the requirements for the UPI system will be detailed. Next, the functional and performance capabilities of the UPI system will be described. Finally, the UPI hardware and software design and architecture will be discussed.