Publisher Summary This chapter focuses on the formation of wear in materials and the selection criteria for wear-resistant materials. Some clues regarding mechanisms of wear are to be found in the shape of debris particles produced in a wear process. Adhesive wear is widely accepted as being an important concept, giving rise to irregular-shaped wear particles. Plastic deformation and fracture processes occur during wear; the starting yield strength or hardness of the surface, or the hardness it attains by deformation without fracture are the important characteristics. The development of the subsurface crack that produces the de-lamination form of wear may be the result of a fatigue mechanism. In surface degradation by corrosion, the technical or economic solution to failure of the surface by wear may well be one of localized treatments, rather than manufacturing the whole item from a wear-resistant material, which might not give the overall properties required or would be more expensive.