Annual, rarely perennial, monoecious or dioecious herbs. Leaves alternate, petiolate. Inflorescence usually spicate, sometimes condensed, terminal or axillary. Flowers mostly unisexual, each subtended by a bract and a pair of bracteoles; perianth segments (tepals) mostly 2–5, occasionally absent, herbaceous or scarious; stamens 3–5, free; ovary superior, unilocular; stigmas 2 or 3. Fruit a 1-seeded utricle, indehiscent or circumsciss; seeds vertically compressed.
About 60 species, most from tropical and temperate regions, chiefly the Americas but almost cosmopolitan; about 20 species in Australia, 5 native.