Expert systems are a branch of the computer science of artificial intelligence. Their ability to mimic experts by applying their domain knowledge has led to the construction of a number of medical applications. A brief resumé of the structure and the processes involved in constructing knowledge-based expert systems is given. Reasons are given for the failure of these successful programs to be widely implemented. It is to be expected that improvements in other areas of artificial intelligence will make them more widely acceptable to the non-expert.