Slipboundary conditionno-slipslip The no-slip boundary condition at a solid–liquid interface is at the center of our understanding of fluid mechanics. However, this condition is an assumption that cannot be derived from first principles and could, in theory, be violated. In this chapter, we present a review of recent experimental, numerical and theoretical investigations on the subject. The physical picture that emerges is that of a complex behavior at a liquid/solid interface, involving an interplay of many physicochemical parameters, including wetting, shear rate, pressure, surface charge, surface roughness, impurities and dissolved gas. In Sect. 19.1 we present a brief history of the no-slip boundary condition for Newtonian fluids, introduce some terminology, and discuss cases where the phenomenon of slip (more appropriately, this may often be apparent slip) has been observed. In Sect. 19.2 we present the different experimental methods that have been used to probe slip in Newtonian liquids and summarize their results in the form of tables. A short presentation of the principle and results of molecular dynamics simulations is provided in Sect. 19.3, as well as remarks about the relation between simulations and experiments. We then present in Sect. 19.4 an interpretation of experimental and simulation results in light of both molecular and continuum models, organized according to the parameters upon which slip has been found to depend. We conclude in Sect. 19.5 by offering a brief perspective on the subject.