Expert systems attempt to capture the knowledge of a human expert and make it available through a computer system.[Duda, R. O. and Shortliffe, E. H., ``Expert Systems Research'', Science, vol. 220, no. 4594,15 April 1983, pp. 261-268.]
Expert systems are expected to achieve significant actual performance in a specialized area that normally requires a human expert for successful performance, e.g, medicine, geology, investment counseling.
Expert systems have been some of the most successful applications of A.I. Since these programs must perform in the real world, they encounter important issues for A.I.: