To develop the physical properties of paper, it is necessary to prepare the cellulose fibres. This is done in wet mills or refiners. Ln these machines, the dilute fibre suspension is forced through a gap between two surfaces fitted with bars that move relatively to each other. Their fibre collection efficiency, together with the cyclic stresses, lead to particle breakage (fibre shortening), fibrillation and hydration. In this article, a model for the kinetics of particle breakage is proposed that links the weight-average fibre length to the energy per unit mass consumed by the solid phase and the average impact intensity. The experimental evidence shows the validity of the assumptions and supports this theory of physical comminution. The raw material-mill combination can be optimized and the shortening effect can be predicted under given mill conditions.