[en] Full text: National radioactive waste repository programmes require large amounts of data to demonstrate safety that increase in number, type and quality as these programmes proceed through the successive stages of final repository development, i.e., siting, characterization, construction, operations and closure. As a repository takes decades to develop, long-term data management is a critical aspect for guaranteeing that future generations will be able to understand the decisions carried out today. Metadata, “data about data”, are a fundamental tool of modern data management. They are used to search information in data records; to store the context with data; and to support their archiving and preservation. The OECD/NEA RepMet is an initiative devoted to bringing about a better understanding of the identification and administration of metadata to support national programmes in managing their radioactive waste repository data in a way that is both harmonized internationally and suitable for long-term management and use. After exploring several metadata concepts and their application in non-nuclear fields (e.g., geospatial, records management), the RepMet group started developing data and metadata libraries for topics relevant to radioactive waste repositories. RepMet is currently working on a library for waste packages ready for disposal in repository. (author