The friction and wear behavior of steel (soft, medium, and hard), titanium, aluminum, copper and copper beryllium sliding on a wide hardness range of steel plates was measured in both vacuum and air. The wear mechanism was identified for each test by visual and metallographic inspection and by microhardness traverses below the wear interface. Four mechanisms were observed and their occurrence correlated with alloy couple, hardness variations, and vacuum conditions. In addition to prow formation, which has been discussed in the literature, three distinct types of rider wear mechanism are herein identified and defined as severe, intermediate and mild rider wear. The mechanisms are: 1) Prow formation: Characterized by a build up on the slider tip of work hardened wear debris (usually from the plate), which separates the plate from the slider. The sliding interface is in the plate below the prow and friction is governed by plate metal properties. 2) Severe rider wear: Metal is worn off the rider and is transferred and cold welded to the plate. The sliding interface is in the rider and friction is governed by rider material strength. 3) Intermediate rider wear: Metal is removed from the rider but little or none is cold welded to the plate. The sliding interface is intermittent between slider and plate, and friction tends to vary with plate metal hardness. 4) Mild rider wear: A small amount of metal is removed from the slider but plate metal oxides are not penetrated to any degree and no cold welding takes place. Damage to rider and plate is minimal. Friction is governed by plate surface characteristics and is independent of plate hardness.