Ribeiro, A.S.; Sousa, M. C.; Lencart e Silva, J. D., and Dias, J.M., 2016. David and Goliath Revisited: Joint Modelling of the Tagus and Sado Estuaries. Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 123-127. Coconut Creek (Florida), ISSN 0749-0208.The Tagus and Sado estuaries discharge in the same coastal region into the Portuguese continental shelf. Several studies focus on the investigation of the complex circulation at the mouth of Tagus or Sado estuaries, however, the interaction between these two systems was never taken into account and were not performed previous studies dedicated to this topic. To study this important issue, numerical modelling is an important tool that allows researching the interaction between plumes under different conditions. Thus, it was developed an implementation of the three-dimensional model Delft3D-Flow integrating Tagus and Sado estuaries and adjacent shelf to investigate the complex interaction between flows. The numerical model was calibrated using sea surface height, salinity and water temperature data, and then applied to research the role of river discharge and wind effects on the plumes interaction. To examine the response of the estuarine plumes to different wind directions, four scenarios of moderate winds were considered blowing from each of the main four compass points. Two markedly different realistic scenarios were chosen: moderate and high Tagus and Sado River discharges. Independently of rivers discharges, the results revealed an intrusion of the Sado plume in Tagus estuary. This intrusion occurred in the bottom layers in all scenarios due to the ambient coastal current, even when the river discharges decreases. The reverse pattern was not observed, demonstrating an unexpected impact of the smaller estuary on the larger.