This paper on Thesauri in Educational Documentation attempts to show by description and analysis the difference in application between thesauri and classification schemes as indexing languages in documentary information systems, and from this to infer possible future developments. First, the differences between the pre-coordination and post-coordination of terms are examined and a distinction is made between the library and documentary systems for indexing literature. Special attention has been paid to the manner of written and systematic presentation of index languages. Next, short, analytic descriptions of a number of existing thesauri or thesauri in preparation serve to introduce the problem of specificity versus universality in classification systems and thesauri. The Eudised Thesaurus is placed side by side with others for this purpose. From the comparison of methods of construction used by the various thesauri, the conclusion is drawn that an increasing integration between classification systems and thesauri may be expected. (Author)