Monoecious trees and shrubs. Stipules present, caducous. Leaves alternate, simple. Flowers of both sexes with perianth segments. Male flowers in catkins or pendent tassel-like heads; perianth 4-6-lobed; stamens usually twice as many as perianth lobes. Female flowers in groups of 1 or 3, in spikes or at the base of the male inflorescence; each group surrounded at the base by an involucre of scales; perianth 4-6-lobed; ovary 3- or 6-locular; ovules 2 per loculus; styles 3 or 6. Fruit a 1-seeded nut, in clusters of 1-3, enclosed by an accrescent spiny or scaly cupule.