Monoecious or dioecious (rarely with bisexual flowers), annual or perennial, freshwater or marine herbs, partially or completely submerged, attached to substrate (in Victoria). Stem developed, reduced or stoloniferous. Leaves basal, whorled or in pairs, sessile or petiolate; veins ± parallel, connected by crossveins. Inflorescence 1–many-flowered. Flowers axillary, actinomorphic or zygomorphic, subtended and ± enclosed by 2 ± united floral bracts; perianth of 1 or 2 whorls of (1–)3 segments, or the inner whorl sometimes with a single rudimentary segment; stamens (1–)2–many, in 1 or more whorls; filaments ± slender or reduced; anthers usually basifixed; carpels (1–)3–6(–15), united (or sometimes, at least adhering); ovary inferior; unilocular; placentation parietal; styles (1–)3–6(–15), unbranched or each once (rarely twice) divided. Fruit usually fleshy, berry-like, the seeds released by decay or by irregular splitting; seeds without endosperm.
About 16 genera with c. 140 species, throughout tropical and subtropical regions, with a few species extending to the temperate regions; 12 genera with c. 35 species occurring in Australia, 7 genera in Victoria.