The Activated Sludge Model No. 2 is introduced as a further development of Activated Sludge Model No. 1. Model No. 2 introduces phosphorus accumulating organisms (PAO) and allows us to simulate the behaviour of biological nutrient removal activated sludge systems. Typical wastewater composition and a set of stoichiometric and kinetic parameters are provided in order to make reasonable predictions. The model has not yet been calibrated or verified in any full-scale application. This report contains a simplified version of the full Model No. 2; the full model is published in the final report of the Task Group (Henze et al., 1994).