The planar entry flow behavior of polystyrene, LDPE, and HDPE has been observed experimentally. The purpose of the work was to determine the cause of vortex growth and explain why this phenomenon occurs in some polymers but not in others. To accomplish this a die was constructed in which nearly any flow geometry could be formed by means of inserts. Flow visualization and flow birefringence experiments were performed using this die with 4:1 and 8:1 abrupt planar contraction geometries for τ_12w up to 7x10⁴ Pa and γ from 1 to 80 sec⁻¹. From these experiments it was determined that vortex growth in a polymer is caused by the resistance to flow under the influence of extensional strain. Although extensional viscosity measurements give an indication of whether or not vortices will form, the flow behavior is best correlated by the ratio of the centerline extensional stress to the downstream wall shear stress, τ₁₁ - τ₂₂ / τ_12w, measured in the entry region. The magnitude of this ratio was approximately 2 for LDPE, which exhibited vortex growth, but was approximately 1 for polystyrene, which did not exhibit vortex growth. Based on the experimental results, the numerical work was directed towards the use of a constitutive equation which could predict the extensional properties of the polymers being modelled. The Phan-Thien Tanner model was found to give adequate rheological property predictions and was used with the penalty finite element method to simulate the entry flow behavior of LDPE and polystyrene. Both qualitative (streamline patterns) and quantitative (extensional stress ratio) agreement was found between the experimental and numerical results. Vortex growth was predicted for LDPE, but incorporation of a zero relaxation time at the corner elements was necessary to increase the limit of convergence to the point where vortex growth was predicted. Support for the relation between vortex growth and extensional properties was given by the numerical results. lt was found that, holding all other rheological properties the same, an increase in the predicted extensional viscosity of a fluid results in the prediction of larger vortices. / Ph. D.