This paper describes the process used to design and evaluate a computer-based tool that guides the development of lesson plans. The tool, PLANalyst, contains a small expert system with a human face that analyzes lessons and gives specific feedback for improvement. The process used to create PLANalyst can be generalized to the development of tools to support any complex intellectual task. PLANalyst runs on Macintosh computers and was constructed with HyperCard. The tool allows easy creation and editing of lessons and provides printed out lesson plans that encourage lessons based on sound instructional principles and realistic estimation of time. It also provides a common language and a tool for team teaching and instructional development. The model on which it is based most closely resembles the nine events of instruction of R. M. Gagne and others. Preparation screens and event screens are illustrated, and the expert system is described. Field tests with close to 200 college students have supported the utility of the tool. The final version, still in development, will incorporate a database of useful teaching techniques drawn from ERIC citations. Six figures illustrate the screens. Two appendixes contain event categories and an evaluation form. (Contains 7 references.) (SLD)