Starting from the properties of the materials constituting waste, approaches suitable for size reduction of tough materials are considered. The rotary shears, rotary shredders, and screw shredders that have recently come into use for preliminary, coarse, and medium-level sire reduction of bulky household waste, scrap wood, used tyres, biological waste, old documents are now widespread. The forces exerted by these machines and their design and important design parameters are examined. A necessary detailed comparison of the three basic types of machine depending upon the nature of the material handled (systematic size-reduction studies) has yet to be performed.