Terrestrial water storage represents the total amount of water stored in all continental storage compartments (ice caps, glaciers, snow cover, soil moisture, groundwater, surface water bodies, water in biomass). The change of Terrestrial Water Storage (TWS) over time balances the budget of the water fluxes precipitation, evapotranspiration, and runoff; i.e., it closes the continental water balance. Monitoring TWS is essential for understanding the global water cycle, managing water resources, and assessing climate change impacts. Declines in TWS reveal emerging drought conditions. Excessive changes in TWS can indicate flood risk due to soil saturation or snowpack melt. Geodetic satellite gravity missions are able to detect mass changes associated with large-scale water movement on or near the Earth’s surface. The time series of Earth’s gravity field changes provided by geodesy are essential for monitoring anomalies in terrestrial water storage related to major flood and drought events at global to regional scales, which in turn are fundamental data for quantifying the global hydrological water cycle. The ability to directly monitor global mass redistribution has proven the essential contribution of geodesy to validating and constraining climate models, supporting climate adaptation strategies, and informing regional and global climate agreements. The following geodetic product is the key to realising, monitoring and applying this Essential Geodetic Variable: