In artificial intelligence, an expert system is a computer system that emulates the decision-making ability of a human expert. Expert systems are designed to solve complex problems by reasoning through bodies of knowledge, represented mainly as if-then rules rather than through conventional procedural code. Expert systems have specific knowledge to the problem domain. Much like other artificial intelligence systems, expert system’s knowledge may be enhanced with add-ons to the knowledge base, or additions to the rules. The more experience entered into the expert system, the more the system can improve its performance. Characteristics of expert systems are Highly responsive , Reliable ,Understandable & High performance.