This book's purpose is to teach people who will be searching or designing text retrieval systems how the systems work. For designers, it covers problems they will face and reviews currently available solutions to provide a basis for more advanced study. For the searcher its purpose is to describe why such systems work as they do. The book is primarily about computer-based retrieval systems, but the principles apply to nonmechanized ones as well. The book covers the nature of information, how it is organized for use by a computer, how search functions are carried out, and some of the theory underlying these functions. As well, it discusses the interaction between user and system and how retrieved items, users, and complete systems are evaluated. A limited knowledge of mathematics and of computing is assumed.