Our recent experiments show(a)that well-entangled polymer solutions undergo a constitutive flow transition when subjected to a shear stress comparable to or higher than the elastic plateau modulus. During such a transition, the measured shear rate may jump orders of magnitude, accompanied by higher flow birefringence and normal stress. More detailed characteristics will be described to elucidate the nature and origin of the observed flow phenomena, which appears to be highly (b)universal and contradicts the current theoretical understanding (c) based on the refined reptation models. (a) P. Tapadia and S.Q. Wang, Phys. Rev. Lett. 91, 198301 (2003). (b) "Transitional Flow Behavior of Entangled Polyisoprene Solutions", A. Philips and S. Q. Wang, presented in the same session at the 2004 March Meeting of APS. (c) J. Bent et al, Science 301, 1691 (2003); R. Graham, A. Likhtman, T. McLeish and S. Milner, J. Rheol. 47, 1171 (2003).