Deciduous, monoecious trees, occasionally shrubs. Lvs alternate, imparipinnate, exstipulate. ♂ fls numerous, usually in pendulous catkins, bracteate; bracteoles usually 2; perianth of calyx only, (1)-3-6-lobed, usually adnate to bracteoles; stamens 3-40, in 2 or more series; filaments short. ♀ fls few, sessile on the axis; bract and bracteoles 2; perianth 4-lobed, ± adnate to ovary; ovary inferior, 1-celled or incompletely 2-4-celled. Ovule 1, erect, basal. Style 2-fid, with stigmatic surface on inner face of branches. Fr. either a drupe with soft exocarp and endocarp hard, intrusive and 2-4-celled at base, or, less commonly, a nut (sometimes winged) and often adnate to enlarged bract and bracteoles. Seed non-endospermic, 2-4-lobed; cotyledons often very contorted and oily.