Fluidized bed jet mills are used for the dry comminution in the mu m range. The application of the jet mill ranges from deagglomeration purposes to grinding of hard materials. This indicates the wide field for the usage of this grinding technique, The comminution effect is based on particle-particle impact in expanding gas streams, A dynamic classifier is integrated in the fluidized bed jet mill. There are two parameters which can be used to control the performance of the fluidized bed jet mill: the speed of the classifier rotor and the pressure of the gas used for grinding. As a characteristic value the ＇＇spin-number＇＇ was defined. The ＇＇spin-number＇＇ is given as the ratio of the circumferential speed of the classifier rotor and the flow-through velocity of the classifying gas in radial direction, Results of limestone grinding were presented. The comminution of limestone represents a grinding process, not just a deagglomeration step. The fineness achieved, expressed by the mean particle diameter, can be modelled as a function of the ＇＇spin-number＇＇ and the pressure of the gas used for grinding. A similar dependency was found for the parameters of the RRSB-distribution, describing the shape and the position of the distribution. The dependencies could be backed-up with experiments performed with an easily grindable rubber chemical. A few experiments performed at the base-points of the working range of the fluidized bed jet mill enable to establish a characteristic field of performance for a given product. The knowledge of this characteristic field simplifies the selection of the optimum operation conditions of the fluidized bed jet mill for a given grinding task. Moreover it enables to include milling processes in more advanced process control systems.