There is an ongoing debate as to whether model-based or case-based diagnostic expert systems are superior. Our experience has shown that the two are not mutually exclusive and, to the contrary, complement each other. Current expert system technology is capable of two reasoning mechanisms, in addition to other mechanisms, integrated into one system. Depending on the knowledge available, and time and cost considerations, expert systems allow the user to decide the relative proportion of case-based to model-based reasoning to employ in any given situation. Diagnostic support software should be evaluated by two critical factor groups: (a) cost and time to deployment, and (b) accuracy, completeness and efficiency of the diagnostic process. In this paper we will discuss the role of expert systems in combining model-based and case-based reasoning to effect the most efficient user defined solution to diagnostic performance.