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Tides are regular rises and falls in sea level caused by the gravitational forces of the Moon and the Sun acting on the Earth's oceans. They are most noticeable along coastlines and in large bays and estuaries, and can also affect connected seas and shallow lakes, though with smaller amplitudes.

The mechanism involves two main gravitational bulges: one on the side of the Earth closest to the Moon and one on the opposite side. The Sun’s gravity also influences the oceans, though its effect is smaller than that of the Moon because of its greater distance. As the Earth rotates, different areas pass through these bulges, producing tides at most locations with a roughly semi-daily cycle. Depending on local conditions, tides can be diurnal (one high and one low per day), semi-diurnal (two highs and two lows of similar height per day), or mixed, with varying high-water magnitudes.

Tidal patterns and their amplitudes vary with geography. The tidal range—the difference between high and low water—depends on coastal shape, bathymetry (depth profile of the seabed), and the geometry of the nearby ocean basin. Weather conditions can modulate tides through atmospheric pressure and wind. Human activities and climate change can influence coastal tidal dynamics, for example via sea-level rise or altered sediment transport.

Tides have practical and ecological significance. They drive nutrient exchange in intertidal zones, influence coastal erosion and sediment transport, and affect shipping, harbor operations, and coastal infrastructure. Technological approaches to harness tidal energy aim to convert tidal motion into electricity through tidal turbines or barrages. Notable tidal phenomena include tidal bores in certain rivers, produced by rapid changes in tide over narrow, shallow channels.