A broad range of well-structured problems--embracing forms of diagnosis, catalog selection, and skeletal planning--are solved in 'expert systems" by the methods of heuristic classification. These programs have a characteristic inference structure that systematically relates data to a pre-enumerated set of solutions by abstraction, heuristic association, and refinement. In contrast with previous descriptions of classification reasoning, particularly in psychology, this analysis emphasizes the role of a heuristic in routine problem solving as a non-hierarchical, direct association between concepts. In contrast with other descriptions of expert systems, this analysis specifies the knowledge needed to solve a problem, independent of its representation in a particular computer language. The heuristic classification problem-solving model provides a useful framework for characterizing kinds of problems, for designing representation tools, and for understanding non-classification (constructive) problem-solving methods. To understand something as a specific instance of a more general case--which is what understanding a more fundamental principle or structure means-i s to have learned not only a specific thing but also a model for understanding other things like it that one may encounter. [13]