Comprehensive rules, guidelines, and examples are provided for use by ERIC indexers and lexicographers in developing and maintaining the "Thesaurus of ERIC Descriptors." Instructions are included for adding new Descriptors and modifying existing terminology; procedures for justifying these actions are provided, as well. Definitions and functional descriptions are given for the general concepts of "thesaurus" and "descriptors" and for the individual elements included in a standard, structured thesaurus unit, i.e., Main Terms, Descriptor Groups, Scope Notes, UF/USE References, Narrower Terms, Broader Terms, and Related Terms. The purpose and use are described for each of the four sections of the ERIC Thesaurus: Alphabetical Display, Rotated Display, Hierarchical Display, and Descriptor Group Display. Discussions are provided on the use and control of educational terminology, both generally and within the context of the decentralized ERIC Clearinghouse network. Other details are included on the historical development of the ERIC Thesaurus, as well as the present-day Vocabulary Development Program that is involving all ERIC components and users in the ongoing activities of Thesaurus development. (JH)