At present, "mechanochemistry" is synonymous with "grinding," according to the views of a significant number of scientists and technologists. Often, one comes across the opinion that "the less the particle size, the better." The cases of considering chemical reactions occurring during pretreatment, as well as considering changes in the ultrastructure of cell walls are extremely rare. Also, the wrong choice of the type of mechanical impact and the equipment used in most cases leads to excessive consumption of electrical energy and reduce economic efficiency. The review presents the currently available published data on mechanically activated processes for the pretreatment of plant materials and shows that when using mechanical treatment, it is necessary to look more closely at the phenomena occurring, rather than reducing everything to the production of fine and ultrafine powders. As a result of mechanical action, active surface radicals can form, hydrothermal chemical processes can occur, and mechanocomposites can form. The role of interphase processes, changes in surface chemistry, related dimensional effects, and the disordering of the crystal structure and amorphization should be taken into account. In addition, the physicochemical insights in mechanical pretreatment make it possible to more efficiently use the energy delivered to the material, and, consequently, increase the economic efficiency of the activation process.