Queries may be enhanced in one of two ways: the user can type in more words or phrases during the session, or the system can offer suggestions. One source of suggestions is a thesaurus --- providing the user with a bank of terms which may be relevant to the query and potentially able to improve the success of the search. The usefulness of a thesaurus depends in practice on its size and detail, whether it is up to date, and whether the terminology it offers is recognised by the user as relevant to his or her own needs. The last point concerns the interaction between the thesaurus construction and the cognitive aspects of the search task --- users may find it impossible to relate the question in their own minds to a list of words or phrases from a different source.