The field of education serves as the basis for this discussion on the use of categories in a thesaurus for information processing and documentation purposes. The author briefly shows how a number of writers concerned with the structure of the field of education, as well as makers of classification schemes, have commented on the value of setting up a system of categories of terms in order to define and make clear their notion of the structure of the field. Included in the discussion are the "Barhydt/Schmidt Thesaurus," the "International Bureau of Education Thesaurus," S. R. Ranganathan's facet analysis, and London Education Classification, and the "EUDISED Thesaurus." (Author/SJ)