Comprehensive rules, guidelines, and examples are provided for use by ERIC indexers and lexicographers in developing and maintaining the "Thesaurus of ERIC Descriptors." Evaluation and decision criteria, research procedures, and inputting details for adding new Descriptors are documented. Instructions for modifying existing Thesaurus terminology are provided, as are procedures for justifying these actions. Details are included on the present-day Vocabulary Development Program that involves all ERIC components and users in the ongoing activities of Thesaurus development. 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 Term, Descriptor Group, Scope Note, 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. Section appendices include a history of the ERIC Thesaurus and some examples of microcomputer templates used for Thesaurus update transactions. (JH)