This paper describes recent research on detection and location of failures in physical systems by application of fuzzy set theory. More specifically, it outlines a procedure by which a human operator can provide a computer a priori deterministic information, in the form of a dynamic model, and fuzzy specification of criteria for failure and corrective action. The computer can then combine the human inputs with its own real-time measurements and simulation to make recommendations to the human operator about what if anything has failed, what is likely to have caused it, and what to do about it. The procedure is suggested for human decision-aiding and not for automatic control at this point. We are applying the techniques to the NASA Hubble Space Telescope, but no details will be given on this application; this paper only outlines our methodology.