The Union Catalog of Art Images (UCAI) is a research and development project located at the University of California, San Diego, and is funded by the Andrew W. Mellon Foundation. The first phase explored the barriers, both practical and theoretical, to creating a union catalogue and tackled problems such as the mapping of non-standardized metadata to a common record and the ingest of those records into a prototype union catalogue. Two of the biggest challenges were record clustering (using automated techniques to identify duplicate or closely-related records and bring them together), and working with hierarchical records (works and their surrogate images). The second phase, now under way and ending in mid-2005, will move the prototype union catalogue closer to production-level quality.