Many hardware/software co-design models have been proposed [7, 2, 5, 6] that attempt to address problems in the hardware/software interface, in partitioning the system between hardware and software, and in finding a uniform method of verifying the system’s correctness. This paper describes the High Order Object-oriented Modeling Technique [4, 3] and its use in specifying and designing a hardware-inthe-loop test environment for the non-trivial application of bulk power systems control. A HOOMT model of the system for the real system and the test system is given. The paper also presents our solution to the mapping of requirements from the real system to the test system. Categories and Subject Descriptors D.2.2 [Software Engineering]: Design Tools and Techniques—Object-oriented design methods; D.2.5 [Software Engineering]: Testing and Debugging General Terms Design, Verification Keywords HOOMT, Hardware/Software Co-design, System Test, Test Development