COST, Harmonizing Global Biodiversity Modelling (HarmBio) http://www.cost.eu/COST_Actions/ca/CA15206. Global biodiversity is declining rapidly, largely as a result of human activities. Effective policy and adaptive management strategies in the face of global change require anticipation of future changes. Mid- to long-term planning will therefore depend, at least in part, on model-based projections. Unlike the well-coordinated climate modelling community, the biodiversity modelling community is currently disparate and largely uncoordinated. Hence, there are no agreed metrics of biodiversity produced as standard output from models, nor are there common datasets used for calibration and validation by modelling efforts. This Action facilitates the harmonization of current models and datasets of terrestrial, freshwater and marine biodiversity to improve the reliability of future projections of biodiversity change. This cross-community initiative aims to accelerate the development of transparent and scientifically robust biodiversity models, through validation, calibration and intercomparison of models and data, and ultimately to enable environmental decision making based on state-of-the-art projections of biodiversity change under various policy options.