The importance of vibration data in determining the condition of rotating machinery is well established in both the aircraft and the heavy equipment industries. Installation of noncontact displacement probes for shaft motion and either velocity or acceleration sensors for bearing cap or foundation motion is standard practice for equipment manufacturers. Automation of the diagnostic evaluation of certain simple faults can be easily implemented. The advances in computer languages in recent years have made it diffiucult to keep pace with the graphical capabilities available to the programmers. One major concern is the content of the knowledge base and the method of modifying the knowledge base or the procedure of evaluating the confidence of a given identified possible cause of a problem. This article addresses the application of one such expert system to a recentvibration problem on a 7-megawatt steam turbine–driven generator located at the power plant of Virginia PolytechnicInstitute.Published version