The large strain behavior of a glassy polymer was studied in terms of compression tests at various rates, while the local deformation was evaluated with a noncontact laser extensometer. The main features of yield and postyielding were described with a constitutive model from nonlinear viscoelasticity, combined with a kinematic formulation for the separation of total deformation into a viscoelastic and a plastic part, respectively. The concept of plastic shear transformations introduced Elsewhere was used to develop a mathematical model for the rate of plastic deformation. The entire experimental true stress-strain curves (including strain hardening and strain softening) could, through this model, be identified in a self-consistent manner.