Groundwater (GW) is the world’s largest distributed freshwater storage for mankind, ecosystems, and is a key resource for industrial and agricultural demands. Due to its fundamental role in the Earth's water and energy cycles, groundwater has been declared as an Essential Climate Variable (ECV) by GCOS, the Global Climate Observing System. However, within Copernicus - the European Earth Observation Programme - there is no service available yet to deliver data on this fundamental resource, nor is there any other data source worldwide that operationally provides information on changing groundwater resources in a consistent way, observation-based, and with global coverage. Therefore, the Global Gravity-based Groundwater Product (G3P) project aims at developing an operational global groundwater service as a cross-cutting extension of the existing Copernicus - the European Earth Observation Programme - portfolio. G3P capitalizes from the unique capability of GRACE and GRACE-FO satellite gravimetry as the only remote sensing technology to monitor subsurface mass variations, and from other satellite-based water storage products that are already part of the Copernicus portfolio, to provide a data set of groundwater storage change for large areas with global coverage. G3P is obtained by using a mass balance approach, i.e., by subtracting satellite-based water storage compartments (WSCs) such as snow water equivalent, root-zone soil moisture, glacier mass, and surface water storage from GRACE/GRACE-FO monthly terrestrial water storage anomalies (TWSA). Compatibility of the observation-based WSCs with TWSA is achieved by a filtering process, where optimal filter types were derived by analyses of spatial correlation patterns. G3P groundwater variations are provided for almost two decades (from 2002 to the present), with the monthly resolution, and at a 0.5-degree spatial resolution globally. In this contribution, we also illustrate preliminary results of the G3P data set and of its uncertainties, as well as its evaluation by independent groundwater data. This study has been run in the context of the European Union’s Horizon 2020 research project G3P (Global Gravity-based Groundwater Product, grant agreement nº 870353).