An analysis of 447 books in arts/humanities, science/technology, the social sciences, and generalities determined how frequently and for what purpose cross-references were used in back-of-book indexes. Of 376 indexes examined, 300 included cross-references. The average index had 32 cross-references per index or 96 cross-references per 1,000 text pages. Of the three major subject areas, cross-references most commonly occurred in science/technology indexes. ‘See’ references were surprisingly dominant over ‘see also’, especially in science/technology and the social sciences. The references made many kinds of links, most often to show synonomy, class membership, and agent/activity relationships. It is encouraging that many indexes have cross-references, but name indexes could have more.