We have developed a continuous theory for grinding of solids in batch and continuous mills which includes only a few parameters, the values of which can be obtained directly from data on a laboratory scale batch mill. This allows an easy scale-up from a laboratory-scale batch mill to an industrial continuous mill. Furthermore, the analysis presented indicates that a characteristic material parameter, the "critical volume", plays an important role on the grinding process in general, and it is the only parameter which is affected by the additives. The performance of industrial mills is due to the influence that additives may have on the value of the critical volume.