Trees or shrubs, often deciduous. Leaves opposite, usually palmately or pinnately divided, rarely entire; stipules usually absent. Inflorescence a corymb, umbel, raceme or panicle. Flowers all or some unisexual; sepals 5 (rarely 4 or 6), free or sometimes shortly fused near base; petals free, equal in number to sepals and often resembling them; stamens 4–12, but commonly 8, filaments sometimes basally united with the prominent nectary disc, anthers basifixed or versatile, opening by longitudinal slits; ovary superior or half-inferior, 2-locular, usually prominently laterally compressed, ovules 1 or 2 per locule, styles 2, free or shortly united, stigma linear. Fruit a winged schizocarp, (a double samara), the mericarps each 1-seeded.
About 110 species mosty from cooler regions of Eurasia and northern America, extending to the tropics in Malesia. Widely grown for ornament. Four species naturalised in Australia.