Herbs, perennial, aquatic or in marshes, with creeping rhizomes. Stems simple or branched, robust or slender. Leaves alternate, distichous, emergent or floating, sheathed at base. Flowers minute, densely crowded in globose heads on upper part of stems and branches; several male and female heads in panicle, raceme, or spike, male heads above female. Perianth segments of 3-6 chaffy elongate scales. Male flowers with 3 or more stamens; anthers basifixed, longitudinally dehiscent; filaments free or partly united. Female flowers bracteolate; ovary 1-loculed or rarely 2-loculed, narrow at base; styles simple or forked, with a unilateral stigma; ovule 1, pendulous. Fruit nutlike, indehiscent.

About 19 species: mainly in temperate and cool regions of the world, best represented in the N Hemisphere; 11 species (three endemic) in China.