Instructional Design Expert (ID Expert) is a prototype instructional design expert system which supports the instructional design phase of instructional system development. It differs from other instructional design toolkits in that it is built using a knowledge engineering approach, and has been implemented as an expert system. The system incorporates both "public" knowledge (in the forms of instructional design theories and models) and "private" knowledge (representing the artistic aspect of instructional design). The project spelled out such "private" knowledge explicitly. This report presents the knowledge representation mechanisms used in ID Expert and related implementation issues. It identifies the five main stages in the development of an expert system (problem identification, conceptualization, formalization, implementation, and testing). The problem identification phase of ID Expert development was performed by senior knowledge engineers from Teknowledge, Inc. The focus of this report is on the remaining four stages. An introduction to the project is provided in the first section, and the second presents a conceptual model of instructional design implemented in ID Expert. Section three presents the results of the conceptualization and formalization phases, and section four describes the results of the implementation and testing phases. (30 references) (GL)