In 2003 the Online Computer Library Center (OCLC) and Research Libraries Group (RLG) established an international working group to develop a common, implementable core set of metadata elements for digital preservation. Most published specifications for preservation-related metadata are either implementation specific or broadly theoretical. PREMIS (Preservation Metadata: Implementation Strategies) was charged to define a set of semantic units that are implementation independent, practically oriented, and likely to be needed by most preservation repositories. The semantic units will be represented in a data dictionary and in a METS-compatible XML schema. In the course of this work, the group also developed a glossary of terms and concepts, a data model, and a typology of relationships. Existing preservation repositories were surveyed about their architectural models and metadata practices, and some attempt was made to identify best practices. This article outlines the history and methods of the PREMIS Working Group and describes its deliverables. It explains major assumptions and decisions made by the group and examines some of the more difficult issues encountered.