The research is involved with the development of a theoretical basis for knowledge based systems in design. A concept of 'design interpretation' is formalized for the definition of desired characteristics of expert systems for design. A new approach for dual mode interpretation between graphic and verbal interaction is proposed and developed. A new representation scheme for complex architectural knowledge termed "The Generation and Refinement Scheme of a Design Prototype" is developed. PRODS - a prototype based expert system Shell for design, which utilizes these ideas is implemented. Hybrid representation, and a knowledge based interpreter which provide communications between the different representational schemes are employed. Two applications of PRODS are developed and demonstrated. PREDIKT - a graphic verbal dual interpretation expert system which is capable of both generation and evaluation, and PROUST - an expert system for the generation and refinement of a design prototype.