Hardware monitoring has proven to be a useful means for measuring the performance of computer systems generally, and is particularly attractive for use on real-time systems due to its attribute of non-interference with system operation. This technique is uniquely able to quantify precisely the interactions between hardware and software, which must be completely understood in these systems. In this paper, we report the application of a commercially-developed hardware monitor to two real-time computer systems. The first was an airborne computer whose basic task was radar data processing. The second system was a more powerful shipboard computer which included a unique, extensive internal performance data gathering facility. The objectives of these projects were to answer several specific performance questions, to demonstrate techniques for software verification and performance enhancement, and to obtain data for system modifications and future designs. A variety of measurements were employed to meet these objectives. They are described in the paper together with our rational in selecting them, typical measurement results, and a discussion of the measurement significance.