Today′s doctors require decision support aids to help them cope with the management of increasing amounts of medical information (records, research advances, new drugs), make appropriate choices and even to substitute in an expert′s absence. Such aids exist in the form of medical expert systems, which are complex computer programs that emulate clinical reasoning. Expert systems consist of a knowledge base in which doctors expertise is encoded and an “inference engine” which manipulates that knowledge. A number of successful diagnostic, management and combined systems are in use but these are a small fraction of the total available. Preventing wider usage are difficulties in evaluation as well as in response time. Significant improvements in resource management can be obtained by the deployment of medical expert systems, so they are predicted to influence profoundly the future of health care in general practice and hospitals alike.