This chapter outlines common mechanisms that contribute to wear, which is broadly defined to be any form of surface damage caused by rubbing one surface against another. Such wear mechanisms include delamination wear, adhesive wear (where adhesion followed by plastic shearing plucks the ends off the softer asperities, typically described by Archard’s law), abrasive wear (where hard particles or asperities gouge a surface and displace material), and oxidative wear (where surfaces react with atmospheric oxygen prior to being worn). Sliding conditions often determine which wear mechanism dominates, with the main factors being temperature, sliding velocity, oxidation, plasticity, loading force, and mechanical stresses. How wear rates respond to changes to these factors can be diagramed on a wear map. The last part of the chapter discusses how transition state theory can describe nanoscale wear by atomic attrition, and how plasticity and fracture occur at the nanoscale.