This paper describes experiments on the wear between a cylindrical metal pin and a hardened steel disk. Under steady-state conditions at light loads it is found that the volume V of material worn away is proportional to the load W, and the length L of path traversed so that V = k'LW. Since the real area of contact A may be written as A = W/Pm, where Pm is a strength property of the pin, the wear equation may be rewritten V = k'LA /Pm = kLA, where k is a constant for the surfaces. This relation suggests that, of the welded junctions formed at the interface and sheared during sliding a constant fraction is detached to form the wear particles. On this View an increase in load produces a proportional increase in the number of welds each of which remains approximately of constant size. This is supported by an examination of the wear particles. This mechanism would seem preferable to the atomic wear model suggested by Holm, which also yields a wear equation of the form V = kLA. At higher loads, in excess of an average pressure about one-third the hardness of the pin, a large increase in the wear rate is observed. It is suggested that this is primarily due to the fact that the true contact area has become such a large fraction of the apparent contact area which is available that a loose wear particle once formed is not able to get away without producing further particles in a self-accelerating process. These results are discussed in relation to the practical problem of running-in newly assembled machine parts.