The successful application of modelling and simulation in design and optimisation rests heavily on having a model which is lifelike. In the case of autogenous (AG) and semi-autogenous (SAG) mills this has provided a significant challenge to modellers, due in part to the interaction between feed characteristics and mill performance and also due to the difficulty in obtaining high quality relevant data front full-scale circuits. As a result the literature contains numerous papers on AG/SAG mill modelling, the majority of which contain little or no data to indicate how well the models predict full-scale mill performance. In an attempt to address this problem a new AG/SAG mill model has been developed which incorporates the capability to respond to changes in most design and operational variables. The model can be used as a stand-alone design tool, to scale-up from pilot data and to optimise existing circuits. It predicts throughput, power draw and product size distribution as well as load size distributions and slurry hold-up. The overall structure of the model and the sub-processes that it contains is described. The model is successfully validated using data from over 21 different full-scale circuits. (C) 2003 Elsevier Ltd. All rights reserved.