For a number of mechanical working processes using a lot of different metals it has been proven that in dry friction plastic deformation is always observed in the contact zone. It has been stated that friction is a result of plastic deformation. However the process of wear is not understood. In the past several investigators tried to interpret it in a thermodynamic way but this more or less failed. At present it is possible to prove that it is started by diffusion. For a very stable tool workpiece material combination we can fundamentally prove that diffusion always takes place. For the tool material it means a change in the chemical composition, leading to a decrease in hardness. Together with the high process temperatures, which lower the hardness of the tool material too, it is possible that the cold workpiece material can be smeared out and taken along with the tool material. The tool thus is worn. From this point of view the influence of the free enthalpy, the thermal conductivity, the hardness, and the ductility of different tool - workpiece material combinations on wear can be clarified. A number of practical examples are given to support this idea.