Herbs, annual, small, submerged in fresh or brackish water. Stem slender, fragile, much branched, rooting at base and lowermost nodes. Leaves subopposite or pseudowhorled, sessile, linear, small, 1-veined, sheathing at base, margin spinulose-toothed; sheaths variously shaped, often auriculate. Plants monoecious or dioecious; flowers minute, unisexual, solitary or few in leaf axil. Male flowers spatulate or rarely not spatulate; perianth (often named "involucre" or "envelope") 2-lipped, membranous, closely applied to anther; stamen 1, anther subsessile, 1-4-thecous, dehiscing at apex. Female flowers sessile; spathe absent or rarely present; perianth persistent. Fruit an elliptic-oblong achene; pericarp thin, membranous, indehiscent, persistent. Seeds without endosperm; testa hard, brittle, pitted; embryo straight.

One of the Fl. China editorial committee, Wu Zhengyi, prefers to keep the Najadaceae separate because of its distinct morphology in having superior ovaries and nuclear endosperms.

About 40 species: cosmopolitan; eleven species (one endemic) in China.