Harmonized forest biodiversity monitoring data in EU forests is currently lacking, which is pressuring monitoring on EU policy outcomes and the achievement of climate and biodiversity goals. Member states collect forest data through, often extensive, National Forest Inventories (NFIs). Most NFIs have, however, started c. two decades ago with a focus on trends in wood resources and less on biodiversity dynamics. This makes NFI baseline measurements susceptible to the so-called ‘shifting baseline syndrome’. In contrast, long-term data with first surveys from the mid-20th century are available from the forestREplot database, which, includes data from more than 5,000 vegetation resurvey plots from c. 80 regions across temperate Europe. The data from both monitoring schemes are not standardized across regions and would greatly benefit from benchmarking against data coming from more frequent and standardized monitoring schemes. The network of 561 permanent ICP Forests-Level II sites, resurveyed every five years since the 1990’s, meets these criteria and allows to understand the uncertainty in the observed forestREplot- and NFI-based trends. ForBioMon aims to harness these three data sources with objectives to (1) obtain a long-term view on biodiversity changes in European forests, (2) improve the understanding of the drivers behind the observed changes, (3) upscale and map forest biodiversity trends, and (4) provide guidance for future forest biodiversity monitoring, by showing the value of including less studied taxonomic groups and key biodiversity drivers. Focus is on quantifying and understanding changes in multiple Essential Biodiversity Variables (EBVs), which are high-level indicators of biodiversity derived from low-level primary observations that are used to study and report biodiversity change. The work is organised in four work packages to address these objectives: WP1) Project coordination and stakeholder collaboration, WP2) harmonizing the complementary NFI, forestREplot, and ICP Forests Level II monitoring schemes and derive a consistent suite of plant EBVs from these sources, WP3) estimate trends, understand drivers and map the plant EBVs, and WP4) explore options for improved monitoring of forest biodiversity by accounting for overlooked taxonomic groups (e.g. bryophytes, lichens, fungi) and additional data sources (remote sensing, historical land-use). Stakeholders, such as the (national) monitoring authorities, are key in WP1, for selecting the EBVs to be focused upon, deciding on the way the biodiversity trends should be presented and communicated, and providing guidelines for future monitoring. On the EU policy perspective, ForBioMon’s outputs are directly responding to the ongoing public consultation in programs, part of the EU Forest Strategy for 2030. The overall transnational effort of mapping forest biodiversity change on the European level may thus help to improve the monitoring schemes and their uses.