Over the past ten years aeration systems development in the United States has centered around fine pore diffuser upgrades to existing activated sludge plants. A variety of older aeration systems have been replaced with new diffuser systems which include full floor coverage systems, composed of ceramic or plastic discs, ceramic and plastic tubes, and membrane tubes or discs. This manuscript reports on aeration system upgrades at six different full scale treatment plants. Seven different devices were evaluated using the off-gas analysis technique. Diffuser cleaning methods were also evaluated. Results are presented which show the economic incentives for upgrading and the increased plant capacity which resulted from increased aeration efficiency. It was found that the effects of process operation, such as step feed, F/M, MLSS concentration, mean cell retention time and air flux had a major impact on transfer efficiency. The effects of diffuser fouling and cleaning methods are presented.