In spite of the continued interest in the yield, flow and fracture behavior of glassy homopolymers and the important role that crazing plays in their ultimate mechanical response, the basic understanding of the molecular level processes that govern craze initiation still remains murky. Here we revisit some early experiments of Argon and Hannoosh of the 1970s on craze initiation in homo polystyrene under combinations of both deviatoric shear stress and mean normal stress and also take note of more recent computer simulations of ultimate plastic flow and cavitation response of model glassy polymers. We then formulate a new craze initiation model and compare its predictions with the earlier experiments to find much better agreement over a wider range of applicability that sheds new light on this hitherto inadequately understood phenomenon. (C) 2011 Elsevier Ltd. All rights reserved.