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A new theory of metal transfer and wear is suggested in this paper. The essence of the theory is as follows: Metal transfer and wear take place at points of actual contact. The interfaces of the high spots that actually make contact are roughened as the result of plastic deformation when they carry normal load. The mechanical interlocking effect of these roughened interfaces is the primary cause of metal transfer and wear. Due to the mechanical interlocking effect of the roughened interface, and the strain‐hardening that accompanies plastic deformation, the application of a tangential force will break one of the pair of the contacting high spots a certain distance away from the interface rather than at the original interface. A secondary cause of metal transfer is the adhesion or the diffusion process which takes place during the temperature flash that occurs during breakage. If the adhesive force is very weak and the diffusion process is not rapid enough to cause the sheared‐off peak of the high spot to become a blob of transferred metal, the small piece of metal sheared from the high spot can leave as a loose wear particle. This proposed theory explains the welding of the sheared‐off peak to its opponent high spot as the consequence of friction, whereas, in the ``welding'' theory of friction, welding is considered as the cause of friction. Most metallic surfaces in ordinary atmosphere are always covered by a surface film. The effect of surface contamination on metal transfer, wear, and the shear component of friction is discussed. Difficulties encountered in applying the ``welding'' theory to explain the friction of and metal transfer between contaminated surfaces where metallic adhesion is absent, are obvious. Experimental support of this new theory is given here. It includes as direct evidence (1) the roughening of the interface as the result of the plastic deformation, (2) the perfect matching at the roughened interface, which gives - a strong mechanical interlocking effect, and (3) a definite region of severely strain‐hardened metal near the interface.