When you go into a library you usually have one of two purposes in mind. In some cases you may be looking for a particular book or journal, report or recording. You know that you want the latest Jeffrey Archer novel, an article about global warming in last week's New Scientist, or the film version of Romeo and Juliet directed by Zeffirelli. Generally you have enough information about the author, title or source of the item for the library staff to help you locate it. Even if you are lacking some details, you know that you don't want the latest Barbara Cartland novel, a piece on global warming in last week's Woman's Weekly, or the film version of Romeo and Juliet directed by Luhrmann. Finding what you want in this case is called known item retrieval, because you already know about the specific work that will meet your needs. Normally the item can be traced using the author's name or the title of the work, or some combination of elements from these.

In many other cases you don't want any particular item, but you do want some information about global warming, or 17th-century drama, or how to grow petunias. You're not bothered who wrote the book or article, what it is called, or who published it, as long as it contains relevant information. The library may have lots of material that will meet your information needs, but none of it can be retrieved using an author, title or publisher, because these are not known.

In order for anyone to find material about a given topic it is essential that the individual books and other items in the collection have had their subject content identified and recorded. Looking for information based on the content of documents is known as subject retrieval, subject searching, or subject access. In the majority of libraries and information services this formal identification of the subject content, and subsequent searching, is done by using a classification scheme and/or a system of subject headings.

In this book we shall be looking at some of the problems that arise when we try to organize documents by their subjects, and how classification schemes and subject headings are applied to the items in a collection in order for readers (and library staff) to find what they want in the most effective way.