This study examines the surface defects known as sharkskin and the physical mechanisms that cause its appearance. Authors of previous papers who studied different polyethylenes attribute the occurrence of sharkskin, either to the initiation of slip at the fluid-wall interface or to the existence of local tensile stresses at the die exit. To test the slip hypothesis, the present authors studied the flow of a linear low density polyethylene (LLDPE) through capillaries of different geometries. The results obtained show clearly that experimental methods for determining slip velocities do not give conclusive evidence of the existence of slip at the wall for the flow of the LLDPE used, under conditions that would normally give rise to sharkskin. The observations obtained using silicone fluids of different molecular weights and an LLDPE suggest that this phenomenon results from the cracking of the fluid at the die exit, due to the high tensile stresses in that region. Such an explanation is supported by flow birefringence measurements and by numerical modeling results published in the literature.