This will be the third edition of the highly successful "Text Information Retrieval Systems." The 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. This third edition will be updated to include coverage of the WWW and current search engines. In many cases, examples of non-web searching will be replaced with web-based illustrations. Coverage of interfaces, various features available to assist searchers, and areas in which search assistance is not available will also be covered. In addition, the book will have a web dimension which will include relevant material available online, to be used in conjunction with the text. It is a follow-up to the award winning 2nd Edition. It focuses on computer-based system but basic principles can be applied to any information seeking context.