Global and regional sea level change is studied by assimilatingsatellite altimetry, sea surface temperature and hydrographicdata intoan ocean general circulation model.The free sea surface of the model changes due to steric expansion,mass exchange through thesurface and local redistribution by internaldynamic processes. These processes are analyzed separately inorder to interprete themeasurements correctly and understandthe underlying reasons, which are prerequisite for predictingsea level change.The global mean sea level of the model is most sensitive to theglobal ocean mass budget, i.e. inflow by rivers, meltingice,precipitation and evaporation. This budget is poorly knownin comparison to the evolution of the volume of the ocean asdetermined from altimetry.It seems more reasonable to use the assimilation results toimprove the estimates of freshwater fluxesthan to usemeasured fluxes to determine the oceans mass change.