Union catalogues have had an important place in libraries for many years. Their use has been little investigated. Recent interest in the relative merits of physical and virtual union catalogues and a recent collaborative project between a physical and several virtual union catalogues in the United Kingdom led to the opportunity to study how users use and view union catalogues and to consider the views of librarians towards union catalogues. The research reported here used a mix of recorded searches, interviews and focus groups to understand user behaviour and views of union catalogues. The findings revealed a diversity of search approaches, the influence of the Web on user expectations and an almost complete lack of awareness of union catalogues on the part of potential users. (Contains 11 tables.)