The application of piezoelectric actuators for active vibration control (AVC) of rotating machinery is examined. Theory is derived and the resulting predictions are shown to agree closely with results of tests performed on the air turbine driven-overhung rotor. The test results show significant reduction in unbalance, transient, and subsynchronous responses. Results from a 30 hour endurance test support the AVC system reliability. Various aspects of the electromechanical stability of the control system are also discussed and illustrated. Finally, application of the AVC system to an actual jet engine is discussed.