A survey of the development and experience with knowledge-based systems applied to power systems in Europe, including expert systems used in operation, is presented. The technical aspects of power system operation are briefly reviewed, the role of computers in power systems is discussed, and the motivation for investigating knowledge-based applications in this context is identified. The structure of the research and industrial organizations involved in the development and implementation is described, and the characteristics of main application areas are discussed. Future expert system development and implementation are discussed