Machine breakdowns and maintenance delays are large contributors to lost production and increased operating costs in the mining industry. The U.S. Bureau of Mines is investigating the applications of expert system technology to diagnostic and predictive maintenance for mining equipment. This will help increase mining productivity and decrease operating costs, by reducing the frequency of equipment failures and repair times. The Bureau has begun this effort by applying expert system techniques to the diagnosis of hydraulic system problems on a continuous mining machine. This paper discusses the development of the hydraulic diagnostic expert system.