ESA's first Earth Explorer Core mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) will measure high-accuracy gravity gradients and provide global models of the Earth's gravity field and of the geoid. An improved gravity-field map will provide insight into the physics and dynamics of the Earth's interior and an accurate global geoid model will contribute to an improved understanding of ocean circulation, which plays a crucial role in energy exchanges around the globe and hence climate. This mission will also lead to a greater insight into sea-level change and to a global unification of height systems.