This paper describes the work done in a 0.69 m diameter wet continuous SAG mill and with various simulation models to predict the performance of full-scale SAG mills. The use of such small diameter mills has been the subject of controversy among various authors. There are, however, several advantages offered by testing on a smaller scale, making it attractive to pursue this line of investigation, These are: lower sample requirements, ability to use drill core to test ore variability, and ease in emptying the mill to monitor changes in its content under different operating conditions. The ore used for these tests amounted to no more than 500 kg per test series. Comparisons made with full-scale SAG mills showed very good agreement in the areas of net specific energy consumption and the product size distribution. The use of 80 mm balls and a feed top size ranging from 80 to 125 mm appear necessary to achieve this agreement. All the characteristics of full-scale SAG mills were reproduced in the small SAG mill, and were easily measured. Predictions made by different simulation programmes were compared. A conclusion drawn from the work is that small-scale SAG mills can be used to accurately design SAG mill circuits, provided that the ball size and feed size of the samples used is large enough. Scale-up can be done directly from the results using familiar formulae. Simulation can enhance the value of the results and be very useful in testing a number of options. The approach is advantageous equally to scale-up for small operations, where large samples may not be available for pilot-plant testing, and for design of large scale milling circuits, where overdesign could lead to major cost penalties.