An ordered approach to expert system design is offered by a shell in which modularization of all data and inferential structures is a feature. The system is completely menu driven, and all APL code is transparent. The user can move easily between design mode and run mode at any time, without any loss of current data. Complex systems can be systematically developed from constituent subsystems. Uncertainty is accommodated by the use of fuzzy logic. All aspects of the expert system are editable at all times.