Abstract: Active balancing technology can reduce vibration in time bythe adjusting the unbalanced state during the operation of a rotating machine, so as tokeep it in a stable state. Early balancing devices, dating from thelate of the 19$^{\rm th}$ century, include those of liquid type, ring type, ball type and pendulumtype; beginning from the middle of the 1960s one sees sprayingtype, electro-erosion type, electro-ray type, laser type and electrochemicaltype; and beginning from the late of the 20$^{\rm th}$ century,motor type and remote control type; and modern balancing devicesinclude those of hydraulic type, electromagnetic type and electromagnetic bearingtype. The mechanical structures and operating principles of each type ofon-line automatic balancing devices used in the rotating machine at home andabroad are summarized in this paper. The hydraulicand electromagnetic balancing devices are particularly addressed.The current research findings on the on-line active balancing technology arediscussed, and the engineering application examples are given. The experiments using theelectromagnetic active balancing device to solve the vibration problem of the flexible rotor with bending fault and the hypergravity machine rotor areintroduced. The on-line active balancing technology has been applied to themachining tools to improve the working accuracy, such as the grindingmachine . The active balancing devices have been used in thelarge-sized rotating machines with tremendous economic returns, suchas steam turbines, fans and compressor engines. Some key problemsincluding the balancing actuator with high reliability and the controlstrategy with high efficiency have not been completely solved.