Global reference frames are fundamental to describing the position and motion of the Earth, other celestial bodies, and artificial satellites. They provide the basis for assigning time-dependent coordinates to points, observing instruments, and objects on or near the Earth’s surface, and for modelling both the static and time-varying components of the Earth’s gravity field. Geometric reference frames, such as the Terrestrial Reference Frame (TRF) and Celestial Reference Frame (CRF), are indispensable for satellite-based Earth observation, and positioning, navigation, and timing (PNT) applications. They also enable the detection and quantification of changes in the Earth’s shape, surface deformation, surface kinematics, and rotation, including variations associated with the solid Earth, ice sheets, oceans, and hydrological systems. Physical reference frames, such as the Gravity Reference Frame (GRF) and Height Reference Frame (HRF), are essential for determining heights relative to mean sea level and for monitoring gravity-driven changes within the Earth system. In particular, they support the detection and analysis of mass redistribution processes occurring within and between the atmosphere, hydrosphere, cryosphere, geosphere, and biosphere. Together, global reference frames constitute the foundation of all applications that rely on the determination, management, and use of georeferenced data (geospatial data). These applications include land administration, surveying, engineering, mapping, geographic information systems (GIS), positioning and navigation services, telecommunications, transportation, and many other sectors of modern society. Beyond these practical applications, global reference frames provide the scientific basis for quantifying and understanding Earth system processes, including plate tectonics, sea-level change, post-glacial rebound, climate change, earthquakes, volcanic activity, and hydrological variability. They are also indispensable for the development and operation of monitoring and early warning systems designed to mitigate the impacts of natural hazards and support informed decision-making for sustainable development. The following geodetic products are key to realising, monitoring and applying this Essential Geodetic Variable: