A method to improve the efficiency of greenhouse climate control based on the framework of optimal control theory is described. By exploiting a dynamic model of the greenhouse crop production process, information on auction price, operating costs of the climate conditioning equipment and outdoor climate conditions, the optimal greenhouse climate control scheme balances basic costs against revenues for operating the equipment. In a greenhouse experiment (using lettuces) the behaviour of conventional greenhouse climate control by the grower was measured. Then, in simulation experiments, optimal control strategies were calculated for the same conditions (outdoor climate, auction price, energy price). The results support the conclusion that a considerable improvement in the efficiency of greenhouse climate management is possible. This improvement may well exceed 15%.