Numerical models for the simulation of tidal waves in estuaries have "become a standard tool of coastal engineers. Before they can he applied to practical problems, they have to be calibrated against nature. The basis for calibration is normally a representative set of tidal curves (natural field data), which has to be reproduced by the numerical model. Generally, the calibration is performed by empirically tuning certain parameters, until a fairly good agreement between measured and calculated quantities is obtained. In most cases, this is a "trial-and error" process which may become very time-consuming, and which strongly depends on the intuition and experience of the user. In order to make the process of calibration more effective, and to ensure that the best possible agreement between nature and numerical model is achieved, a calibration method has been developed to determine the parameters of the numerical model automatically by means of a mathematical method of optimization. The method is applied to a one-dimensional numerical model of the Elbe River.