Experiments were performed in a sliding plate rheometer with a high density polyethylene to determine the conditions for the onset of slip and the relationship between slip velocity and shear stress. It was found that melt slip occurs at a critical shear stress of approximately 0.09 MPa in both steady and transient shear tests. The effect of the presence of a layer of fluorocarbon at the interface on both the slip velocity and the critical shear stress for the onset of slip, was also studied. Exponential shear was used to study the effect of shear history on slip. Both steady state and dynamic models for the slip velocity are proposed that are consistent with the experimental observations. Results of oscillatory shear experiments suggest that melt slip is a physicochemical process in which the polymer–wall interface undergoes continuous change during successive cycles.