In much heavy industrial equipment, power transfer is primarily provided by a hydraulic system. Reliable operation of a hydraulic circuit can only be ensured through proper design, effective maintenance and continuous monitoring. Each of these areas requires the expertise of well-trained and experienced individuals. Unfortunately such individuals are not always available. However, the recently introduced technology of “expert” or “knowledge-based” systems can make these individuals' expertise available in the form of a computer program.As part of a long term project in the application of expert systems to hydraulic circuit design, a prototype program has been developed which can successfully configure a variety of circuits including multiple load and add-on circuits. This is currently being expanded to include sequenced circuits, steady-state circuit analysis, which would include component sizing, and dynamic analysis. Another of the authors' current projects is to examine the feasibility of developing an “introspective machine” where on-line condition monitoring is coupled with an expert system to assess the health of a hydraulic system and advise the user of any potential problems, breakdowns etc. Recently a small demonstration program has been written to demonstrate the feasibility of producing an expert system for the fault diagnosis and troubleshooting of the hydraulic system of a mining machine. This paper will summarize the authors' work in the application of expert systems to these three areas.