An expert system, the Computer-Aided Systems and Control. Analysis and Design Environment (CASCADE) has been implemented for the design of multivariable controllers for linear plants, using the linear quadratic Gaussian/loop transfer recovery (LQG/I,T11.) design methodology [1,2,3]. The expert system was implemented using a novel shell, DECIDE [4,5], which uses a decision expression methodology and incorporates explanation and backtracking, and linkage with the numeric and graphics software necessary for control system analysis and design. CASCADE was evaluated by generating trial designs for several realistic applications in the following areas: High voltage DC power transmission, robot manipulator path tracking, large structure active stiffening, aircraft stability augmentation, wind power cogeneration, bioreactor control, and steam power plant control. Since these designs were performed by graduate students, this provided an evaluation of the CASCADE implementation as both an expert system-based design package and an aid in education. We found that not only was the expert system useful for computer-aided design and education; it also served to demonstrate several subtle fallacies in the theoretical design methodology. Our surmise is that these fallacies had remained undiscovered because of the significant investment required in the mechanics of the design process in previous work. The expert system allowed attention to be focused-on a codification of design knowledge rather than on the algorithmic details.