Deciduous or evergreen, monoecious trees. Leaves alternate, simple, petiolate, with stipules at least when young. Male flowers in pendulous or stiff catkins or one to numerous in capitula; stamens 4–40. Female flowers in small groups of one to few, surrounded by numerous small scales; ovary 3- or 6-celled, with 2 ovules per cell; styles 3–9; stigmas linear. Fruit a cupule formed from the fused scales, containing 1–3(–6) nuts.

Contains 8 genera and about 900 species, worldwide but absent from tropical and South Africa and east tropical South America.

Deciduous, monoecious trees. Buds fusiform, acute at apex. Leaves alternate. Male flowers in tassel-like heads on a long pendulous peduncle; perianth campanulate, 4- to 7-lobed; stamens 8–16. Female flowers usually paired, surrounded below by long scales; styles 3, long. Fruit a cupule covered with projecting scales or prickles, 4-valved, woody, containing 1 or 2 triquetrous nuts. Germination epigeal.

Contains 10 species in north temperate regions and Mexico.