This paper presents a novel concept for calculation of optimal maintenance strategies for equipments that degrade over time. The concept is based on a dynamic mathematical model that is used in a look-ahead strategy to assess the effect of different future operational strategies. The optimum is given with respect to minimum costs for maintenance and the costs for not performing maintenance. The main idea is to illustrate that given these costs; the optimal strategy might be to wait a certain time before maintenance is per-formed. By this, considerable costs might be saved compared to performing the maintenance immediately or wait too long time before the maintenance is performed. In practice, rough estimates are often satisfactory for such decisions. Hence, the accuracy of the model behind the calculations might not be critical. The methodol-ogy is illustrated for a case of internal leakage in anti-surge valves for compressor control.