This chapter contains sections titled: Introduction Purpose of Modeling Elements of Activated Sludge Models Transport Processes and Treatment Plant Layout Aeration Components Processes Hydraulic Patterns Presentation of Models Mass Balances Rates Component Participation The Activated Sludge Models Nos. 1, 2 and 3 (ASM1, ASM2, ASM3) Activated Sludge Model No. 1 (ASM1) Activated Sludge Model No. 2 (ASM2) Activated Sludge Model No. 3 (ASM3) Wastewater Characterization Model Calibration Computer Programs Use of Models References