®®Expert systems (or knowledge-based systems) allow the scarce and expensive knowledge of experts to be explicitly stored into computer programs and made available to others who may be less experienced. They range in scale from simple rule-based systems with flat data to very large scale, integrated developments taking many person-years to develop. They typically have a set of if-then rules which forms the knowledge base, and a dedicated inference engine, which provides the execution mechanism. This contrasts to conventional programs where domain knowledge and execution control are closely intertwined such that the knowledge is implicitly stored in the program. This explicit separation of the knowledge from the control mechanism makes it easier to examine knowledge, incorporate new knowledge and modify existing knowledge.