Trees, usually deciduous, often dioecious. Leaves opposite, imparipinnate (in Victoria), rarely 3-foliolate or simple, usually serrate. Inflorescences axillary or terminal, many-flowered panicles, appearing before or with leaves. Flowers bisexual or unisexual, small, pedicellate; calyx campanulate with 4 small lobes or calyx absent; petals 2–6, united at base in pairs, or absent; stamens 2, inserted at base of corolla, anthers linear-elliptic; ovary flask-shaped, style terminal, short, with 2 elongate stigmatic lobes. Fruit a 1-seeded, winged samara.
About 70 species, from the Northern Hemisphere, particularly temperate Asia and North America; 5 species naturalised in Australia.