THE application of the viscosity concept to this technically important field is presented. Discussed first is film lubrication which occurs when the film thickness is greater than 10−5cm. This mechanism is chiefly controlled by the viscosity &eegr; of the liquid. The basic mathematical theory is developed, and it is shown how this adequately describes the experimental facts. Next boundary lubrication is discussed, a type occurring with stable layers less than 10−5cm thick. This mechanism is essentially a plastic flow within the boundary layer. The chief experimental data are described together with their technical implications.