Beaches form as waves deposit sand and other sediments and wind pushes these sediments inland, creating a beach and accompanying dunes. The size and shape of a beach can wax and wane dramatically throughout its lifetime, as beaches are influenced by tides, weather, winds, and man-made objects on and around the beach. Therefore, it is possible to track seasonal and yearly changes on many beaches, and these changes are sometimes used to keep track of larger weather trends in the area.

Potentially, a beach can form anywhere where the ocean comes into contact with the shore. Beaches form over millennia, starting with wave-cut terraces caused by the ocean's action on the shore, along with geological changes created by shifting tectonic plates. Over time, a flattened area emerges where the waves hit the shore, and a beach starts to form. When beaches form, the larger they get, the better their foothold; while small beaches may vanish entirely in some years, big beaches will generally remain intact.

Beaches are what are known as deposition landmasses, meaning that they are formed by deposits of sediment and other materials. These sediments take the form of sand, stones, and other materials which wash out from inland regions into the ocean; sediments are also created through erosion of the ocean bottom, and the deposition of dead marine animals and plants. Some of these sediments settle to the ocean bottom, but many more are picked up and carried in currents which eventually come close enough to the shore for waves to pull the suspended sediments back onto the land.

As waves push sand and sediment up the beach, they inevitably leave some sediments behind, especially when the tide recedes. Coastal winds then push the sediments up beyond the reach of the waves, creating the start of a beach. Ancient beaches are often surrounded by dunes created from sediments, reflecting a dramatic shift in topography created when these beaches form.

In addition to being pulled up by the wind, sediments can also move through a process known as saltation, when they actually bounce up the beach out of the reach of the waves. The processes of deposition and saltation can be accelerated by storms and other severe weather, causing a beach to grow or shrink depending on which direction the sediment is moved in. Storms often lead to the deposition of large amounts of sediment which may slowly erode away in the months following the storm; in other cases, entirely new beaches form in response to dramatic weather.