This article is a brief inrroduction to one type of knowledge based system the expert system. The discussion centres on the production rule or rule based system (RBS) as, to date, this is by far the most common structure for expert systems. The three major parts of any expert system, the know/edge base, the inference engine and the user interface, are discussed. The use of expert system she//s and expert system building tools can considerably assist the development of an expert system. Attention is drawn to the importance of the knowledge elicitation process and the role of the new discipline of knowledge engineering.