Friction and wear processes have a common origin at minute contact points where either cold welding or surface penetration and cutting occur. Wear can be classified roughly into four types: adhesive wear, abrasive wear, chemical wear, and galling wear. Consideration of the effect of load on real contact area leads to the principle that wear is proportional to the load and inversely proportional to the hardness of the softer of the sliding pair. Abrasive wear decreases to negligible amounts when the abraded surface approaches the hardness level of the abrasive. The mechanism of wear of ceramics is different from metals. It occurs by microfragmentation. Friction of dry surfaces is a function of shear strength of contact junctions and hardness. Friction is independent of apparent area of contact but is extremely sensitive to surface chemistry.