‘The tide in shelf seas’ describes progressive waves, standing waves, and what happens when a shelf sea is in resonance, using the example of the Gulf of St Vincent off the south Australian coast. It also considers the effect of Earth rotation and tides in shallow water, where the rare feature is double high water or double low water. The great ocean basins are bordered by shallow seas lying on the continental shelves. Shelf seas are generally less than 200 metres deep and vary in width from almost nothing to hundreds of kilometres. It is in these shallow seas and the rivers that flow into them that the most spectacular tides are found.