MESS is an expert system to support a decision at the American StockExchange to open an investigation of insider trading. The system was developedwith three experts and extensive prototyping. MESS illustrates multiple roles for anexpert system; it contributes to decision making by seeing that all relevant factorsare considered, stimulating discussion of important criteria, and providing an audittrail for regulatory review The system also serves a training role for a position withhistorically high turnover