An on-line expert system for fossil fuel power plants, Heat Rate Degradation Advisor, or HEATXPRT, is being developed. This expert system will be used to automate the interpretation of plant operating data using available on-line plant physical and performance data, along with manual input of additional measurements and observations when necessary. This expert system will operate on a microcomputer and will interface with existing plant data acquisition and/or thermal performance monitoring systems, which presents a variety of design challenges. Several significant features of this expert system are discussed in this paper, including the design of a modular expert system architecture, design of a consistent user interface, development of a standard specification for information transfer, application of sensor validation techniques, and development of a knowledge base suitable for the interpretation of on-line data.