A lumped parameter torsional-bending model of a spur gear pair with backlash and time-invariant mesh stiffness is used to examine chaos. It can be shown that chaos typically exists in a lightly loaded and lightly damped gear pair when excited by the static transmission error. These results are qualitatively, but not quantitatively, similar to the studies reported on Duffing's oscillator. When radial clearances in rolling-element bearings are introduced, a three degree of freedom nonlinear model is used to identify two different routes to chaos: (i) period-doubling route and (ii) quasiperiodic route. Predicted time histories, phase planes, Poincaré maps, and Fourier spectra will be illustrated to examine these. Limited experimental data. as available in the literature will be cited in evidence. [Work supported by NASA Lewis Res. Ctr.]