Slip in entangled polymer solutions was studied using a plane-Couette shear flow cell. On a bare silica surface, a log-log plot of slip velocity vs shear stress displayed several power law regimes. Slip behavior was observed to be a strong function of the chemical nature of the surface. On a low-energy surface obtained by grafting octadecyltrichlorosilane onto a silica surface, the slip velocity was found to be essentially proportional to the shear stress to the first power up to a moderate stress. At higher stresses, slip velocity was found to be a strong function of shear stress on both bare and grafted silica surfaces. At high stresses, the evidence of enhanced concentration fluctuations was detected. Concentration fluctuations were observed to be strongly influenced by the interaction between the polymer and the surface. A scaling model is developed to estimate the frictional drag on a probe polymer chain (N-mer) pulled by one of its ends through an entangled polymer(P-mers). This model takes into account changes in the probe chain conformation and relaxation processes of the probe and the surrounding chains. The model is extended to study slip of an entangled polymer over a weakly grafted solid surface, and its predictions are found to be in good agreement with the experimental results.