Monoecious trees or large shrubs, evergreen or deciduous, with scaly buds. Leaves alternate, simple, with caducous stipules. Flowers unisexual, usually in different inflorescences on the same main twig. Staminate flowers arranged in many-flowered catkins or in tassel-like heads, rarely solitary or in 3's, each flower in the axil of a bract, usually with a green, simple perianth of 4-8 divisions and with as many or twice as many stamens; filament distinct; anthers basifixed, 2-celled; rudiments of gynoecium present or 0. Pistillate flowers solitary or in groups of 2-5 with a common involucre composed of concrescent bracts accrescent at maturity and forming a cupule; perianth usually of (4-)6 tepals arranged in 2 whorls; ovary inferior, usually 3 (-6) -celled, with 2 anatropous, pendulous ovules in each cell; ovule with 2 integuments; styles as many as cells. Fruit (due to abortion of the other ovules) a 1-celled, 1-seeded nut, surrounded or enclosed by the scaly or spiny cupule. Seed with no endosperm; cotyledons thick; coat membranous.