An activated sludge model is a mathematical model to describe the complex biochemical reaction of micron germs and pollutants. This paper reviews the biomathematical models of activated sludge in our country and abroad, including the Activated Sludge Model No.1(ASM1), the Benchmark Simulation Model No.1 (BSM1), the Activated Sludge Model-Secondary Sedimentation Tank (ASM-SST), the Activated Sludge Model-Aeration System Simplified (ASM-ASS), the Activated Sludge Model No.2 (ASM2), the Activated Sludge Model No.2d (ASM2d), the Activated Sludge Model No.3 (ASM3) and the Fully Coupled Activated Sludge Model No.3 (FCASM3). The existing problems of these activated sludge models are discussed. Some suggestions about activated sludge models are made. It is pointed out that the reaction mechanism of the activated sludge model should be studied in order to simplify the model; experiments should be conducted to determine parameters in the activated sludge model; and the activated sludge model software development should be encouraged, with applications in the industrial waste water engineer design and the water quality monitoring.