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Abstract: The experimental study of structural vibration is often performed to determine the modal parameters of a structure or to verify the theoretical models and predictions. The first phase of this research involved the experimental determination of the modal properties of a rectangular steel tank with different levels of water. The natural frequencies obtained from the experiments were compared to those calculated by the analytical models. In the second phase, a procedure for computing hydrodynamic pressures in rectangular tanks is proposed. This procedure considers the effect of tank wall flexibility in determining the hydrodynamic pressures produced by the impulsive response. Based on a two-dimensional model of the tank wall, a dynamic time-history analysis was carried out. The results were compared with other models based on the current design practice codes and standards, which use a lumped mass approach. The comparison shows that, in most cases, the lumped mass approach overestimates the base shear. The effect of wall flexibility on wall displacements and base shears are also discussed.