A beach is an expanse of sand or pebbles along a seashore that is washed by the tide and waves. Erosion often reduces beach width in winter when wind and wave action are more powerful and frequent. In summer, gentle wave action transports sediments that replenish the beach with sand. Most beach sand consists of light-colored quartz and feldspar sand grains—the result of weathering and erosion of rocks such as granite.

Some beach sand comes directly from shoreline erosion, but much is created by the action of rivers flowing to the sea. Most beach sand contains fragments of smoothed and rounded shells from clams and other marine creatures. Tropical beaches often consist entirely of shell and coral fragments. Beaches in areas of volcanic activity can be black, where the sand was created by erosion of volcanic rock.

There are several types of beaches found along the coasts of the United States. Common to northern California and Oregon is the narrow stretch of sand bounded by rolling surf and a rocky cliff mainland, called a spit. These beaches are located where waves break upon a coast of hard bedrock with little available sediment. The swash (wave uprush) and the return backwash carry pebble-size fragments ashore, while finer sand is washed to sea. Numerous beaches on the Atlantic Coast are of the spit type, such as New York's Coney Island where a narrow shoal extends seaward. This type of beach is created by wave action over a lengthy period.

Beaches are a major attraction for coastal living. About two-thirds of the world's population lives within a narrow coastal belt that extends landward from the ocean's edge. In the United States, many large cities are located near or on an ocean shoreline or alongside one of the Great Lakes.