Mammillaria, native from southwestern North America to the Caribbean, Colombia, and Venezuela and especially concentrated in Mexico, is a large and complex genus of about 150 species. Its species are favourites for pot culture and are commonly grown outdoors in warmer climates. Mammillaria species are small globose to cylindroid plants. The tubercles (projections) are arranged in definite spiral rows and are not connected into ribs. Areoles (specialized buds) on the tips of the tubercles bear spines.

Many small flowers, in many hues, are borne between tubercles in rings around the plant below the tip. In some species the petaloids are striped with a different colour or a darker shade. Some species, such as M. saboae, have showy flowers larger than the plant. Several species, including old woman cactus (M. hahniana), are valued for their white woolly appearance.