This paper reviews work undertaken to better understand the performance of physical and virtual union catalogues for the purposes of resource discovery and retrieval within the JISC Information Environment, as under the auspices of the CC-interop project. System-level variations in the ability of systems to process search queries were noted and the impact of disparate cataloguing and indexing practices on recall and precision were also investigated. Recommendations for improving interoperability (including semantic interoperability) are proposed
