The yield stress of amorphous glassy polymers depends strongly on the strain rate, temperature, and hydrostatic pressure. On the other hand, the yield criteria, valid for metals, do not describe this behavior. Therefore, the yield phenomenon of the polymeric glassy state must be mainly a nonlinear viscoelastic effect. In this work, an analysis based on the concept of activation volume tensor, described elsewhere, and using fundamental equations of nonlinear viscoelasticity, is presented. It has been confirmed that this consideration leads to a satisfactory description of the postyield behavior, including strain softening and strain hardening. The strain rate and temperature effect have also been modeled for various rates and temperatures.