This fourth edition is the result of a significant revision of the world-renowned textbook on expert systems and software development using the CLIPS expert systems language toolkit. The book includes information related to two main areas; the first half of the book outlines the theory of expert systems and shows what place expert systems occupy in the entire volume of computer science, and the second contains information on programming using the CLIPS language. Another new tool described in this publication is the object-oriented COOL language. At the beginning of the book there is a separate introduction to the topic of artificial intelligence, the volume of which is sufficient to study expert systems.