Often the information available to describe the deep structure of the lithosphere under sedimentary basins is restricted to local deep seismic profiles and sparse information from seismology. Here 3D gravity modelling provides a helpful tool to derive the spatial distribution of densities at depth. Using relationships between seismic velocities and densities established in lab experiments we derive density models the cumulative 3D gravity response of which is consistent with observed gravity. A particular strength of 3D gravity modelling is that layers at different depths produce a gravity signal of characteristic wavelength and that the non-uniqueness of 2D gravity models is severely reduced if the true 3D influence of mass is considered.