Expert systems, also known as intelligent knowledge based systems (IKBS), are computer programs which act as decision-support systems. They are currently being applied to a number of medical domains, most notably diagnosis and treatment planning. Their function is to assist the medical practitioner by giving ready access to the levels of skill shown by experts in a particular field. Much research effort has been expended but few systems have reached routine medical use. This paper presents a tutorial introduction to expert systems in medicine, explaining the basis of the technology, its current limitations and its prospective uses.