EMERGE is an expert system designed as a medical decision making aid. It is machine-independent, and is implemented in standard Pascal. It has modest memory requirements, and can operate on a microcomputer. EMERGE is rule-based, and its initial application is the analysis of chest pain in the emergency room. The knowledge base is maintained separately from the consultation program. Thus the application area can be changed without any modification to the software. This paper describes the control structures and rule searching procedures used in EMERGE.