Have you read a book from beginning to end and then realized that none of the ideas in the book can be directly used in the real world? This may be acceptable in some fields, but not in the area of expert systems, which is inherently practical. You cannot really understand what expert systems are unless you have implemented one. In writing this book we have tried to ensure that readers get practical knowledge, which will help them not only to understand expert systems technology, but also to implement expert systems. We have been teaching courses on expert systems for many years. We have evaluated many books for use with these courses. We found that most books currently available do not provide a satisfactory introduction to expert systems programming. Books we looked at were either too full of detail, or too abstract. Basic concepts were completely left out in some books. It would be valuable to combine practical work using an expert system shell (such as CLIPS, a shell developed at NASA), with the study of this book. While the case studies and examples used in the book are primarily based on Vidwan, they can easily be converted to run on other rule based expert system shells.