Cactaceae (cacti) are perhaps the easiest recognized family of plants in the US; features include thick, fleshy stems (sometimes woody), a dense or light covering of spines that grow from areoles, and large, brilliantly colored flowers. They range in size and shape from tiny discs less than an inch wide, to the giant saguaro that can reach a height of 60 feet.

Stems may be cylindrical, spherical or flattened, while the surface can be smooth, ridged, or tubercled. Leaves are generally absent, but a few genera such as opuntia do produce short-lived leaves at the tip of new stems. Spines may be accompanied by short, thin bristles (glochids). Flowers are radially symmetric, formed of many overlapping petals, many stamens and one style, topped by a lobed stigma. There is usually one flower per areole.