Since their first application in the late 1950s, autogenous grinding (AG) and semi-autogenous grinding (SAG) mills have provided the modern mining industry with the workhorses used for most high-throughput comminution applications. Secondary crushing is a means to debottleneck the AG/SAG mills when processing coarse-competent feed. In the early years of AG/SAG milling, secondary crushers were used in the flow sheet to manage feed size for smaller diameter AG/SAG mills and allow high capacity. As bigger shell diameters and larger motors were adopted, secondary crushing options were rarely considered in greenfield flow sheet design but instead regarded as an option to deconstrain an undersized AG/SAG mill when treating high competency or coarse feed. Feeding AG/SAG mills with secondary crusher products changes the breakage behaviour and classification of the ore in the mill when compared with mills fed with primary crusher products. This requires changes to the ball and total mill filling levels, process control, shell liners and discharge system designs. This paper reviews the last 50 years of published data on comminution circuits that use secondary crushing before AG/SAG mills. The key objectives are to assess:  options for optimising the impact of critical size fractions in the mill feed  the opportunities to reduce the grinding media consumption, therefore minimising the overall Greenhouse Gas Emissions (GHGe) footprint for high-throughput applications  the impact of ore competence on optimum mill load (rock and steel) composition for maximising throughput and project value.