The rapid proliferation of online databases has been accompanied by an equally rapid increase in the number of thesauri, particularyly those used by bibliographic databases. This guide is designed to assist users of online databases identify the relevant indexing vocabularies. It identifies the indexing vocabularies used in specific online databases, the syndetic structures employed in specific thesauri, and the arrangement of descriptor displays. Selected for inclusion in this guide are thesauri used by databases that are widely available through the large commercial retrieval services and thesauri used by large databases. In general, the thesauri selected are in the English language. The 122 entries are arranged alphabetically by title. Five indexes, including title, personal name, organization, subject, and database, complete the work.