Abstract: An explanation system for use in an expert system is described. The rule base used in the inference engine of the expert system is divided into groups of rules called rule classes. With each rule class, three types of explanations are associated: strategy explanations, reason explanations and inference explanations. The rule classes are arranged in a hierarchical explanation tree structure. Each time a first rule is fired within a rule class, a frame is created into which the explanations relating to that rule class are copied. After the completion of the reasoning process, the frames can be used to generate an explanation of the reasoning process.