The U.S. Bureau of Mines is conducting research on the application of expert systems technology to mining equipment diagnostics, to increase mining productivity and efficiency. Specifically, an expert system is being developed for diagostic maintenance of the hydraulic system of a continuous coal mining machine. Through the use of such a system, lengthy and costly delays due to equipment failure and repair can be significantly reduced. This type of equipment diagnostic system may be applied to the electrical and mechanical systems as well as other mining machinery. This paper discusses the functional requirements of the expert system, the approach to the problem, the application domain, a description of how the present system works, its software design and sensor interface, and the future plans for the continued development of this and other expert systems in mining.