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Studies of the surface damage caused by the sliding of clean metals on one another show that penetration and distortion occur to some depth beneath the surface. Micro-examination shows that welding of the metals takes place quite readily even at low speeds of sliding when the surface temperature-rise due to frictional heat cannot be very high. In some cases the welded junctions may pluck out portions of the harder metal. These and other results have led to a more quantitative theory of metallic friction. It is suggested that in general the frictional force between clean metal surfaces is made up of two parts. The first is the force required to shear the metallic junctions formed between the surfaces; the second is the ploughing force required to displace the softer metal from the path of the harder. By using steel sliders of various shapes and sizes on a soft metal like indium, these two factors have been estimated separately, and it is shown that an approximate calculation of the friction between metal surfaces may be made in terms of the known physical properties of the metals. The effect of surface contamination is also discussed.