FIS, short for Fault Isolation System. is an ongoing research project at the Navy Center for Applied Research in Artificial Intelligence, a branch of the U.S. Naval Research Laboratory. The focus of the work is a model-based expert system shell capable of acquiring from a user a description of a piece of electronic equipment. called a unit under test (UUT). This description is later used by FIS to perfonn such diagnostic functions as recommending the next "best" test to make on the UUT during a fault isolation sequence and estimating fault probabilities after each test is made. These and other capabilities make FIS a powerful tool that can be used in a variety of diagnostic settings, as discussed in Section 1.2. FIS has been developed primarily with large-scale analog-hybrid electronic systems such as radar and sonar systems in mind, but it is applicable at least in principle to any human­ engineered system with discrete replaceable components. FIS is written in LISP and has been under development for several years. Earlier versions of the FIS prototype system are described by Pipitone [1,21.