Expert systems are very useful and probably the most fruitful products of applied artificial intelligence. Expert systems, however, are very· expensive to develop. Powerful construction tools are indispensable to construct, modify and maintain a practical expert system. GUESS/l is a domain-independent expert systems shell that captures and enhances the strengths of its predecessors while at the same time overcoming.their limitations. GUESS/l gives a strong emphasis on human engineering, language generality, diversity of data representation and control structures, programming and run-time environment, database construction facilities and security, and many other aspects that are related to the ease of development and maintenance of expert systems. / Master of Science