The German partner of the "Estimating the Circulation and Climate of the Ocean" (GECCO) consortium provided a dynamically consistent estimate of the time-varying ocean circulation over the 50-year period 1952-2001. The GECCO synthesis combines most of the data available during the entire estimation period with the ECCO/MIT ocean circulation model using its adjoint. The adjoint model is used to iteratively reduce the model-data misfit: By sweeping backward and forward, the control vector is systematically modified through the Lagrange multiplier method. Together with the initial temperature and salinity fields, the surface wind stress and net heat flux, the net surface freshwater flux is also part of the control vector; i.e., it is being adjust so that the model best simulates observed ocean parameters. The data used as constraints during the assimilation include several satellite data sets, surface drifter velocities, in-situ hydrographic temperatures and salinity profiles as well as WOCE hydrographic sections.