Plants annual or less commonly perennial herbs, monoecious, glabrous but sometimes appearing minutely scaly or yellowish-glandular spotted at high magnification. Stems simple to much branched, often rooting at the nodes. Leaves opposite or sometimes appearing whorled at the stem tip, sessile or short- to long-petiolate. Leaf blades simple, entire, linear to spatulate, the bases mostly connected around the stem by a narrow herbaceous membrane. Stipules absent. Flowers imperfect, tiny, solitary or in small clusters of 2 or 3 per leaf axil, the staminate and pistillate flowers usually together in the same axil, in some species subtended by a pair of minute bracts. Perianth absent. Staminate flower(s) maturing before the adjacent pistillate flower(s), consisting of 1 stamen, the slender filament attached at the anther base. Pistillate flower(s) consisting of 1 pistil, the ovary superior, of 2 carpels, somewhat flattened and 4-lobed, each carpel with 2 ovules. Styles 2, elongate, often persistent at fruiting, the stigmatic region toward the slender tip. Fruits depressed-elliptic or more or less heart-shaped in outline, the ovary separating into 4 achenelike nutlets, these oblong-elliptic to kidney-shaped or obovate in outline, flattened and sometimes narrowly winged, the surface with a very fine network of ridges (observable only with magnification), light brown to brown. About 50 species, nearly worldwide.

Some species of water starwort are used horticulturally in outdoor pools and ponds. The herbage is eaten by waterfowl and some species of fish. The seeds are dispersed by water and by animals, and they reportedly survive passage through the digestive system of ducks. Studies have shown that species of Callitriche have developed an unusual breeding system where pollen germinates within intact, unopened anthers, and the pollen tubes grow downward inside the filament and then through the vegetative tissue and into adjacent pistillate flowers to effect pollination (Philbrick, 1984). This presumably helps to ensure abundant seed production in populations where extreme and rapid environmental fluctuations occur. Also, this helps to explain observations of seed production in plants whose stamens appear to be rudimentary and incapable of dehiscing to release pollen.

As with many groups of aquatic angiosperms having reduced floral structures, the relationships of Callitriche have been controversial. Traditionally, the most widely accepted alignment was as a separate family, Callitrichaceae, close to the Euphorbiaceae (Steyermark, 1963), presumably because of superficial similarities in fruit morphology. Studies in ovule and embryo development suggested a placement as a separate family close to the Lamiaceae and Verbenaceae, which has been followed more recently by a number of botanists (Cronquist, 1981, 1991). The most recent evidence from molecular studies has classified Callitriche into the expanded Plantaginaceae. For further discussion, see the paragraph under the family description above.