This paper examines the factors influencing the failure of lubricant films. It is shown that in the state of elastohydrodynamic lubrication substantial damage does not occur and in this region surface active agents have no noticeable effect. Substantial damage only occurs when a large fraction of the load becomes unsupported by hydrodynamic action, i.e. failure is from the boundary state. It is shown that the magnitude of the surface deformation under the applied load is a major factor in breakdown. When the deformation is elastic, the solid surface films, e.g. of oxide, remain intact and even a poor liquid lubricant provides sufficient protection against the build-up of damage. The transition temperature, i.e. the temperature at which the friction rises, is then high. When the deformation is severe, the transition temperature is much lower. The effect of load and surface finish on the transition temperature is discussed.