Since the first edition of this book in 1981, Prolog has continued to attract an unexpectedly great deal of interest in the computer science community and has turned out to be a basis for an important new family of programming languages and systems for Artificial Intelligence. In the preceding three editions, the authors have steadily added new material, improved the presentation, and corrected various minor errors to provide a textbook as well as a reference work for everyone who wants to study and use Prolog as a practical programming language. The authors concentrate on teaching "core" Prolog. All examples conform to this standard and will run on the most widely-used Prolog implementations some of which are listed in the appendices with indications as to how they diverge from the standard.