A rock crusher is a machine designed to take large rocks as and reduce them to smaller rocks, gravel, or rock dust. Rock crushers produce aggregates and ready-to-process mining ores, as well as rock fill material for landscaping and erosion control.

Crushing is the first step in converting shot rock into usable products, by taking large rocks and reducing them to smaller pieces. Crushing is sometimes continued until only fines remain. At some operations, all the crushing is accomplished in one step, by a primary crusher. At other operations, crushing is done in two or three steps, with a primary crusher that is followed by a secondary crusher, and sometimes a tertiary crusher or even a quaternary crusher.

Raw material, of various sizes, is brought to the primary crusher by rear-dump haul units, or carried by a wheel front-end loader. Primary crushing reduces this run-of-mine rock to a more manageable size.

Impact crushers break rock by impacting the rock with hammers that swing on a rotating shaft. The practical use of impact crushers is limited to soft materials and non abrasive materials, such as limestone, phosphate, gypsum, weathered shales. Impact crushers achieve the highest degree of reduction.