We report new evidence of secondary flow during plane Couette shearing of entangled polystyrene solutions and an entangled polyisobutylene melt. The secondary flow is shown to be driven by normal stress imbalances at the free edges of the materials, and is accompanied by spatial variations in birefringence. In the polymer solutions, the secondary flow, though unmistakable, is weak and is only apparent from tracer particle visualization experiments using a video microscopy technique. In the polymer melt, however, secondary flow is much stronger and causes gross shape changes in sheared samples that are easily visualized with the naked eye. We also show that several details of the plane Couette secondary flow are correctly predicted by a simple differential constitutive equation in the narrow gap approximation. Our findings suggest that the secondary flow’s effect on sliding plate rheological measurements could be minimized by (a) using large plates with narrow gaps, i.e., small sample aspect ratios; (b) performing measurements at low Weissenberg numbers (Wi≡τLγ̇); and (c) employing measuring techniques such as laser birefringence that permit stresses to be determined close to the center of the shearing surfaces of the plane Couette cell. In addition to secondary flow, we find significant levels of slip during steady shearing of entangled polystyrene solutions. The slip, though qualitatively similar to theentangledslippredictions of Brochard and deGennes [Langmuir8, 3033–3037 (1992)], is unusual because at low slip velocitiesVsthe slip lengthsb≡Vs/γ̇ are of the order of 150 μm, which is much larger than expected for entangled slip. Furthermore, slip in the solutions is shown to be nonisotropic with the slip velocity manifesting a similar shear rate dependence to the birefringence. These findings suggest that the slip law for entangled polymers may be more complex than previously thought.