The role of subject and search knowledge in query expansion (QE) is an unmapped terrain in research on information retrieval. It is likely that both have an impact on the process and outcome of QE. In this paper our aim is an analytical study of the connections between subject and search knowledge and term selection in QE based both on thesaurus and relevance feedback. We will also argue analytically how thesaurus, term suggestion in interactive QE and term extraction in automatic QE support users with differing levels of subject knowledge in their pursuit of search concepts and terms. It is suggested that in QE the initial query concepts representing the information need should not be treated as separate entities, but as conceptually interrelated. These interrelations contribute to the meaning of the conceptual construct, which the query represents, and this should be reflected in the terms identified for QE.