Abstract Human expertise should be better understood before the users of expert sytems specify the services needed and expected from such systems. An analysis of expert thinking has been developed to assist in this understanding. The analysis is discussed in the paper under three main headings: Specifications: examples are given of the services users obtain from human experts, in the particular domain of petroleum geology. These services indicate general qualities desirable in a human and, by analogy, in a system. The qualities are listed as specifications for expert system design. A theory of expert thinking: how human experts acquire, understand and use their knowledge (particularly with reference to petroleum geology). The theory identifies a number of human, knowledge-handling techniques which could be implemented in a system to meet most of the user's specifications. Human and system expertise: a comparison suggests that, properly designed and suitably applied, an expert system can help its users make well-informed decisions; failing this, the system may prove dangerously misleading and should not be accepted as a substitute for an accountable, human expert.