This paper proposes an expert system approach for voltage and VAr control. A decentralized control mode is used so that each area performs its proper control using only its local information. It is obvious that the proposed approach does not suffer from the lack of complete power system information which often occurs during emergency operation. Tasks performed by the expert system are presented. The first task analyzes the operating conditions of each area based essentially on its voltage profile and determines appropriate control actions with the existing VAr controllers in the area. The second expert system task consists in finding suitable solutions when the existing VAr devices of an area are not sufficient to resolve voltage problems. The rules required to perform these expert system tasks are identified. Two strategies concerning the control of transformer tap changers and the determination of generator new reactive productions are also discussed. An expert system prototype was developed in C language on Apollo computers. Its performance is evaluated on a real power system consisting of 172 nodes, 72 generators and 42 transformers.