The friction of sliding metal surfaces is often attributed to the formation and shearing of welded junctions between the surfaces. This paper describes a more detailed investigation of the phenomena which occur in the junctions. Experiments were performed with a hemispherically ended copper rider sliding against the surface of a large cylindrical copper drum. Observation of the sliding contact through the microscope shows that most of the time during sliding, the drum and rider are held apart by a wedge of displaced metal from the drum, which becomes trapped between them. Whenever a wedge escapes from between the drum and rider, another wedge forms rapidly to replace it. The wedges are also observed with several other combinations of metals, and with pairs of flat copper surfaces. Sections cut through the wedges and adjacent copper contact members were examined. It is found that the shearing of the metal by which the sliding proceeds occurs in a direction slightly inclined to the drum surface. This causes the formation and growth of the wedges. The shearing is primarily a continuous plastic deformation process somewhat similar to the shearing process by which the chip is generated in the cutting of ductile metals with machine tools. It is possible that the adhesion between the rider and wedge may often be weaker than the junctions usually assumed to be responsible for friction. Wear debris is observed to be produced in a number of different ways.