Information retrieval is a communication process that links an information user or seeker to a computer system that contains data bases or to a librarian, museum curator, fingerprint identification specialist, or whoever is in charge of a collection of what we are calling documents. The communication will normally involve the processing of text, strings of words known to both parties in the process that can be used to describe a document's content and other attributes and link it with a need expressed in similar terms. Even non-text entities, such as musical themes, are converted to alphanumeric symbols to allow effective storage, search, and retrieval. 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 non-mechanized ones as well.