We summarized the experience from three decades of oxygen transfer testing and aeration research at Technical University of Darmstadt to validate the oxygen transfer efficiency of modern fine-bubble diffusers. 306 oxygen transfer tests in clean water of 65 different fine-bubble diffusers, carried out in the same test tank under identical test conditions, were analysed and compared with previous results. As a result, we could show that the performance of fine-bubble aeration systems has increased by 17% over the last three decades. Therefore, modern well designed and operated aeration systems can achieve SSOTE values between 8.5 and 9.8% · m−1. Additionally, a comparison of various diffuser types and diffuser densities was done. Based on the new results an exemplary cost/benefit analyses for a 100,000 PE WWTP shows the calculation of an optimized diffuser density with respect to investment and operating costs.