This paper introduces the structure and characteristic of an expert system based on characteristics of wastewater treatment such as time-variation,nonlinearity,complexity and uncertainty.The paper then analyzes the status of wastewater treatment systems studies and existing disadvantages of expert systems.It shows that expert systems have been applied in nearly all aspects of wastewater treatment systems in other countries,but only initial work has been done in the People's Republic of China.The authors introduce the prospects for and trends in the application and study of expert systems in wastewater treatment.