Sand dunes are hills of sand that are formed by wind or water-driven sand. They typically take the form of a mound, ridge, or hill, and an area with dunes is called a dune system or a dune complex. Dunes grow as grains of sand accumulate, and every dune has a windward side and a slipface. Sand deposits are not restricted to deserts, and dunes are also found along sea shores, along streams in semiarid climates, in areas of glacial outwash, and in other areas where poorly cemented sandstone bedrock disintegrates to produce an ample supply of loose sand.

There are different types of sand dunes, including barchan, seif, star, and transverse dunes. Barchan dunes are crescent-shaped and have horns that point downwind, while seif dunes are long and straight with sharp crests and steep slip faces. Star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound, and they tend to accumulate in areas with multidirectional wind regimes. Transverse dunes are long ridges that are perpendicular to the wind direction, and they are common in areas with strong, consistent winds.

Sand dunes can have a negative impact on humans when they encroach on human habitats. Sand dunes move via a few different means, all of them helped along by wind. One way that dunes can move is by saltation, where sand particles skip along the ground like a bouncing ball. When these skipping particles land, they may knock into other particles and cause them to move as well, in a process known as creep.