The purpose of the present article is to provide a perspective for computational predictions related to plastic instabilities such as bulge, neck and shear band formation and their propagation in polymeric materials under a variety of deformation conditions. Constitutive models of polymeric materials are developed for glassy polymers and phenomenological models, which duplicate the experimentally observed characteristic behavior of polymeric materials, and physical models, which are developed by using the molecular chain network theory, are discussed. Since the instabilities propagate under almost steady state conditions without significant change in force, the special attention should be given to the computational simulations, which is briefly discussed. Instability behaviors such as the onset of bulge, neck and shear band and their propagation are presented with illustrative examples of computational simulations.