The DDH operationalises the “Diamond Open Access” concept through comprehensive parameters and functionalities for data collection. It makes new facets of diamond publishing visible, such as governance and mission. The DDH strengthens Diamond OA publishing of journals from the public and/or institutional sector, owned and operated from within academia or researchers. It aims to create the most comprehensive list of diamond journals in Europe for stakeholders. Readers & authors, Research Performing Organisations, and funders can assess the nature of Diamond Open Access journals and their publishers. Diamond OA journals become visible within a larger collection of institutional journals, highlighting the diamond model. Publishing and Institutional Service Providers can help their journals get their data aligned, improved, and harmonized for broader dissemination, thus reaching recognition for their diamond effort. Aggregators can reuse the index to retrieve data from diamond journals compliant with their requirements. DDH also provides a trusted observatory of diamond publishing channels for policy making and further improvement of the publishing sector.