Abstract: In this paper, the progress in nonlinear dynamics of gear driven systems inthe past twenty years is discussed, especially the gear dynamic behaviorwith consideration of the backlash and time-varying mesh stiffness of tooth. Thebasic concepts, the mathematical models and the solution methods for thenon-linear dynamics of geared systems are then reviewed. The critical issuesin the further research on the nonlinear vibration in gear transmissionsystems are also discussed.