The three pillars of geodesy. The changes in Earth's shape (geokinematic), gravity field and rotation, i.e. the "three pillars" of geodesy, provide the conceptual and observational basis for the reference frames required for Earth observation. Moreover, these "three pillars" are intrinsically linked to each other as they relate to the same unique Earth system processes. Today, the space-geodetic techniques and dedicated satellite missions are crucial in the determination and monitoring of geokinematics, Earth's rotation and the gravity field. Together, these observations provide the basis to determine the geodetic reference frames with high accuracy, spatial resolution and temporal stability.