Waves are formed mainly by winds, but they can also result from movements of the seabed (tsunamis), ships moving through the water (wakes), and currents. Waves are usually described by their height (the distance between crest and trough), length (the distance between two consecutive crests) or period (the time between two wave crests passing a fixed point). The height of a wave produced by wind depends on the wind strength (wind speed), the length of time the wind blows (duration) and the distance over which the wave travels (the fetch). In the English Channel, waves from the south-west are often largest because the winds are stronger from the Atlantic, the weather systems last several days and the fetch may be over 4000 km. In contrast, even with the same wind speed, waves from the south have a fetch of less than 150 km. Their wavelength or period is usually short. As waves travel away from their source, they spread out and their wavelength increases. Waves with longer wavelengths or periods are known as 'swell'.

As they travel into shallower water, waves and swell become steeper and 'break'. The angle at which waves reach the beach affects how much sediment can be transported along the shore.

Tides result from the gravitational forces of the earth, moon and sun. They have very long wavelengths compared to wind waves. Because the positions of the moon and sun relative to the earth change daily, monthly and seasonally, tides have daily (diurnal), monthly and annual patterns. The tides with the greatest range between high tide and low tide are known as 'spring tides' and 'neap tides' have the smallest range. On the Dorset coast, tidal ranges at Spring Tides are less than 3 m. Tides are 'semi-diurnal' where high and low tide occur roughly twice in every 24 hours. Dorset has semi-diurnal tides, but east of Swanage high water is longer than low tide and has two peaks (a double high tide). In contrast, a double low tide occurs in Weymouth Bay. Tides produce currents, which can move loose sediments across the seabed.

When waves break, they can move any loose particles in beaches or exert pressure on the rocks of the cliffs and rock platforms. The rate of erosion of the rocks depends on their strength, chemistry, water content and position at the coast. Rock strength is affected by the cohesion of their materials, the presence or absence of joints, bedding planes, and faults and the angle at which the rock layers (strata) slope (geological structures such as folds). Rainfall, groundwater and waves all affect landslides. The strength of the waves and the protection by beaches or man-made structures also affect erosion.

Some rocks, especially the Portland Stone, are very resistant to erosion, but weak sands and clays, such as the Oxford Clay, are easily eroded and can retreat over one metre annually. Because of this differential erosion, the Dorset coast is characterised by many headlands and bays, with beaches of sand or pebbles.