A diagnostic expert system generator that provides an environment for easily developing, testing and verifying knowledge bases is described. GENESE has been designed to be used by physicians without any particular background in computer science. Four paradigms are offered: a decision-tree editor, a propagation editor, a rule editor/browser, and a C-like programming language. The learning time required to use the system correctly ranges from one hour (for the decision-tree editor, since decision trees are familiar to physicians) to three or four months (for the programming language and the rule editor). However, the learning task is included in the development of the knowledge base and is therefore perceived as a constructive activity.