This paper reviews the basic steps, often overlooked, in the design of activated sludge systems on the basis of recent concepts introduced for process modelling. Firstly, it evaluates the necessary data base for a rational design approach, emphasizing the required wastewater characterization. Secondly, it underlines the significance of functional relationships between reactor biomass, volume and the sludge age recommended as the major design parameters. Then, it defines excess sludge production with specific reference to VSS/SS ratio and to the importance of the influent fixed solids concentration. Finally, it deals in detail with the oxygen requirement of the system, outlining different modelling approaches to be used for such an evaluation and emphasizing the pitfalls to be encountered when using BOD5 parameter for this calculation. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.