The Julius Kruttschnitt Minerals Research Centre (JKMRC) has been involved in the study and modelling of industrial autogenous (AG) and semi-autogenous (SAG) mills for over 20 years. Recent research at the JKMRC has developed a new AG/SAG mill model which is based on charge dynamics. The model relates charge motion and composition to power draw and size reduction. Size reduction is described by considering impact and abrasion/attrition as separate processes. These are linked to the energy available in the mill, the charge size distribution and the relative motion of the grinding media. Ore specific energy-breakage relationships are described using laboratory data which are obtained from breakage tests over a wide range of energies and particle sizes. Slurry transport is described using the JKMRC’s latest model which incorporates the effects of grate design. This paper describes the overall structure of the model together with its main sub-processes.