Abstract Operation of air classifiers, particularly in closed circuit with grinding mills, is described. Performance measures are reviewed and an improvement of the AIChE standard method of calculating the selectivity curve is proposed. Approach to performance optimization is derived from the fundamentals of air classification. The computations of power requirements for classical air separators and for modern high efficiency machines are compared. Trial-and-error calculations for optimizing the complex heat balance of a grinding mill in closed circuit with an air classifier are discussed.