Computerized decision making aids have been developed for use in certain medical specialties over the last decade. These programs are designed to give expert-level advice within their limited domains. Two major components are required in the design of these systems: development of software and development of a knowledge base to handle the specific medical application. The advantage of the knowledge-based approach is that the application can be changed by replacement of the knowledge base, without requiring changes to the software. In this paper,EMERGE, a medical decision making system, originally developed for analysis of chest pain in the emergency room environment, is described. EMERGE uses production rules for representation of its knowledge base. The method by which this system can accommodate new rule bases for other applications is emphasized, as well as the transportability of the system.