Waves were measured over tidal cycles on a sloping, basaltic shore platform at Scots Bay in the macrotidal Bay of Fundy (large tidal range 13.5 m), and on a horizontal, argillite platform at Mont Louis in Gaspé, Québec (large tidal range 3 m). Video cameras were used to record the height and period of the waves against a series of graduated metal poles anchored along surveyed, shore-normal profiles. Field measurement and theoretical considerations suggest that wave height increases with elevation at Scots Bay, reflecting the occurrence of a gently sloping tidal flat below the midtidal level and higher gradients on the upper than on the lower parts of the platform. Calculated pressures generated within the rock along joints and other discontinuities suggest that wave conditions are suitable for mechanical-wave erosion at Scots Bay. Waves generally break on the low tide cliff at the seaward edge of the platform at Mont Louis, preventing any wave action on the shallow, flooded platform surface behind. During high spring tides, greater water depths allow fairly large waves to cross the platform, but a beach generally protects the cliff foot and, apart from a few upstanding ridges, there is no rock exposed to wave action at the water surface at this elevation. Downwearing by weathering dominates on the horizontal Mont Louis platform, but waves would still attack the cliff during high tides if there were no protective beach, which suggests that the platform may have been cut by waves near the high tidal level and subsequently lowered by weathering to its present elevation.