Deciduous trees or shrubs, monoecious; bark dark brown to chalky white, smooth. Buds sessile, with several imbricate scales. Leaves ovate to deltoid, or elliptic to suborbicular, serrate, rarely lobed, often glandular on lower surface. Male flowers produced in autumn, in crowded pendulous catkins with numerous overlapping bracts, each bract subtending 3 flowers; perianth of 2–4 scale-like sepals; stamens (1–)2–3(–4). Female flowers produced below male catkins, in crowded, erect catkins, with numerous bracts, each subtending (1–)3 flowers; carpels 2. Fruit a 2-winged samara.
About 60 species from the Northern Hemisphere; 1 or 2 species incipiently naturalised in Victoria.
Identification to species level in this genus can be complicated, several commonly grown species are highly variable, and hybridisation is known to occur in both cultivated and wildly growing plants.