This report presents the results of an investigation of comminution in tumbling mills. Five topics involved with ball- and rod-mill grinding kinetics were studied, and the results were analyzed in terms of phenomenological size reduction and transport models. Continuous rod-mill grinding was found to be characterized by high peclet numbers that permit simulation to be carried out by several methods. The dry-batch ball-mill grinding of mineral mixtures showed that the breakage rates of the components were composition- dependent but time-independent and were normalizable with specific energy. Locked-cycle tests on mineral mixtures showed that a grinding circuit may take a long time to attain steady state, during which time the composition of the mill holdings, and hence the breakage rate of the components, continuously changed. The study revealed that under plant conditions, grinding circuits can be in transience perpetually. The transport of mineral mixtures through drums showed complex size and/or density-dependent segregation, thereby simplifying the modeling of the continuous grinding of complex feeds. Wet-batch rod-mill grinding on single minerals showed that the normalization of breakage kinetics is valid for slurries of constant pulp density at different holdups but cannot be extended to slurries varying in pulp density.