Intact forests harbor large amounts of carbon and unique biodiversity, suggesting that protecting forests may benefit climate change mitigation and biodiversity conservation alike. Yet, forests also provide other essential services, from timber to energy to recreation. Balancing these multiple, sometimes conflicting objectives requires understanding trade-offs and synergies among them. A key question in this context is whether schemes to maintain or increase carbon stocks through forest management actually co-benefit biodiversity. Although frequently promoted, assumptions about such co-benefits have not been rigorously assessed. Forest and CO will test whether policies designed to protect either biodiversity or carbon in Europe’s temperate forests are synergistic or conflicting. Three work packages (WP) will focus on both unmanaged, old-growth forests, representing a baseline for key ecosystem functions, and managed forest, constituting the majority of Europe’s forests. WP1 will build a network of forest researchers to gather existing data on old-growth forests in Europe, and create the first map of their distribution. WP2 will use plot-level data to model the relationship between forest biodiversity and carbon storage, and assess whether this relationship differs between old-growth and managed forests. WP3 will test whether carbon/biodiversity co-benefits vary, when scaled up to broader extents, using satellite products and vegetation-plot databases. This action will provide new insight into the synergies and trade-offs between carbon and biodiversity in temperate forests and develop methods to map these trade-offs. Scientifically, this will contribute substantially to the fields of forest ecology, conservation science, and sustainability science. From a policy perspective, Forest and CO will help to tailor forest management options to jointly foster carbon and biodiversity, thereby contributing to key policy goals on curbing climate change and biodiversity loss.