The present description of the Trondheimsfjord is based on monthly hydrographical data collected between 1963-80, together with data from periods of intensive measurements using a great variety of methods. The estuarine circulation derives mainly from six big rivers that flow into the fjord. A simple hydraulic theory is used to calculate the residence time of the brackish layer. Tidal flows dominate the water exchange above the sill level as well as the turbulent vertical mixing and diffusion that eventually lead to replacement of the deep water in the basins. Current measurements at the sill reveal significant baroclinically driven residual currents. Variations in the vertical stratification outside the fjord are found to be important for the water exchange. A statistical treatment clearly shows that there is a need for long continuous hydrographic series running over decades in order to distinguish natural variations from the influence of human activity.