The continued popularity of Prolog and oflogic programming in general has called for a new edition in which we have corrected a few minor errors found in the previous edition. Since the previous edition of this book, the development of Prolog has proceeded with much vigour and creativity on the part of hundreds of researchers around the world. For example, the International Conference on Logic Programming will meet in 1995 for the twelfth time; the Internet newsgroup comp . 1 ang . pro log exchanges many messages daily; and there are numerous World-Wide Web resources of interest to Prolog users. It is no longer posssible to track the many books on Prolog and implementations of Prolog that are now available. The Prolog idea has been extended to new languages for manipulating sets of constraints, functional expressions, type hierarchies, and object-oriented programs. And yet throughout this time the Edinburgh Prolog syntax and semantics have served as a standard and as a base of stability. In tum, we hope that this book can continue to serve as an introduction to the language and its use. Cambridge, England W. F. C. July 1994 C. S. M. PREFACE TO THE THIRD EDITION We have added new material to Chapter 3 to give an account of up-to-date programming techniques using accumulators and difference structures. Chapter 8 contains some new information on syntax errors. Operator precedences are now compatible with the most widely-used implementations.