This book is about knowledge systems--how to design them, how to use them, and how to program them in a language called Prolog. A knowledge system is a computer program that solves, or helps to solve, problems that would otherwise would have to be handled by a human expert alone. A knowledge system may be an expert system; a program that understands English (or other natural languages); or a combination of the two. It may also use a data management system to store parts of its knowledge. Knowledge systems are used to give advice and solve problems in many areas, such as business, science, technology, and law. The knowledge systems described in this book have been developed at IBM, and have been used for experiments in several subjects, including manufacturing planning, communication network management, evaluation of the ease of use of software packages, and machine translation from one natural language to another.