Improvements in today's maintenance practices are mandatory as more complex and productive machines evolve in the mining industry. By keeping machine reliability high through improved maintenance procedures, overall mining costs can be reduced. The U.S. Bureau of Mines is investigating diagnostic maintenance systems for underground mining machinery. A hydraulic expert diagnostic system and an electric motor predictive diagnostic system have been designed and installed on a Joy 16cm continuous miner. An electrical diagnostic system has been designed and installed on a Joy 14cm continuous miner. Each system is functionally described, and the relative merits of expert diagnostic systems versus algorithmic diagnostic systems are discussed.