The eXtensible Catalog (XC) L ibrary metadata, whether in the form of MARC 21 catalog records or in a variety of newer metadata schemas, has served its purpose for library users by facilitating their discovery of library resources within online library catalogs (OPACS), digital libraries, and institutional repositories. However, libraries now face the challenge of making this wealth of legacy catalog data function adequately within next-generation Web discovery environments. Approaching this challenge will require: n an understanding of the metadata itself and a commitment to deriving as much value from it as possible; n a vision for the capabilities of future technology; n an understanding of the needs of current (and, where possible, future) library users; and n a commitment to ensuring that lessons learned in this area inform the development of both future library systems and future metadata standards.The University of Rochester's eXtensible Catalog (XC) Project will bring these various perspectives together to design and develop a set of open-source, collaboratively built next-generation discovery tools for libraries. The XC Project Team seeks to make the best possible use of legacy library metadata, while also informing the future development of discovery metadata for libraries. During Phase 1 of the XC Project (2006)(2007), the XC Project Team created a plan for developing XC and defined the goals and initial functional requirements for the system. This paper outlines the major metadatarelated issues that the XC Project Team and XC Partner Institutions will need to address to build the XC system during Phase 2. It also describes how the XC Team and XC Partners will address these issues, and concludes by presenting a number of issues for the broader library community to consider.While this paper focuses on the work of a single library project, the goals and functional requirements developed for the XC Project reveal many generalizable needs for metadata to support a next-generation discovery system.1 The metadata-related goals of the XC Project-to facilitate the use of MARC metadata outside an Integrated Library System (ILS), to combine MARC metadata with metadata from other sources in a single discovery environment, and to facilitate new functionality (e.g., faceted browsing, user tagging)-are very similar to the goals of other library projects and commercial vendor discovery software. The issues described in this paper thus transcend their connection to the XC Project and can be considered general needs for library discovery metadata in the near future.In addition to informing the library community about the XC Project and encouraging comment on that work, the author hopes that identifying and describing metadata issues that are important for XC-and that are likely to be important for other projects as well-will encourage the library community to set these issues as high priorities for attention and action within the next few years.