Five facets of background information are intended to be helpful to persons interested in the construction and use of a thesaurus, in the sense of a standardized language for communication with and between information storage and retrieval systems. Based on the Thesaurus of Engineering and Scientific Terms (TEST) as an example: (1) the thesaurus is described and defined, both as to what it is and what it is not; (2) the story of Project LEX shows how a thesaurus is developed, from rules and conventions to published format; (3) the building of a local specialized thesaurus compatible with the broader standard is explained; (4) application of the thesaurus to systems of indexing other than documents is described and (5) experience with the thesaurus is discussed, and guidance outlined on updating and improvement. The authors are members of the American Society for Information Science Thesaurus Workshop Panel. (Author)