In many applications of the material processing industry, frequently size reduction of solid materials for different purposes is necessary. Some size reduction equipment is designed to control the size of the largest particles in its products, but the fine sizes are not under such control. The most common size reduction equipment will be reviewed as follows. Hammer mills provide good performance, high capacity, and need little space, apart from attaining the highest reduction relations for single comminution units. The applications of hammer mills are numerous and cover diverse industrial processes. For example, high-speed hammer mills provided with internal or external classification can handle a variety of very fine dry powders with great efficiency. The rotary knife cutters have a very similar design to the hammer mills, since they have a horizontal rotor that spins between 200 and 900 rev/min inside a cylindrical chamber.