For many years people have been fascinated with the prospects of building machines that can exhibit intelligent behaviour. Research in Artificial Intelligence has led to study experts in a specialised field and explicate the rules used to solve problems in their field; then build machines that apply these rules to the appropriate situations and solve problems in the ways that mimic the experts. This has led to the development of Expert Systems.This book, an English translation of the original French version provides an insight into the theory and practical applications of expert systems. The text matter is class-tested, and in fact is the result of the author`s classroom lectures to the students. The examples from real-life experiences are presented; concepts are defined as precisely as possible and integrated in the general structure. While the approach of the book is pedagogical, the required rigour is present in the book to have a unified version of different concepts. Though it is a compact book, it is self-contained and comprehensive enough for the students to find in one book what is scattered in `bits and pieces`.