Dioecious trees or shrubs. Leaves generally alternate, simple; stipules free, scale-like and soon deciduous or leaf-like and persistent. Flowers in erect or pendulous catkins, each in the axil of a bract. Perianth reduced to a cupuliform disk, or to 1-2 nectar glands. Staminate flowers of 2-30 stamens; filaments free or united; anthers extrorse, longitudinally dehiscent. Pistillate flowers with 1 pistil having 1 or more short styles and 2 stigmas; ovary free, 1-celled, of 2 (rarely 3-4) carpels; placentae 2-4, parietal. Fruit a capsule, longitudinally dehiscing by 2 (rarely 3-4) valves. Seeds small, numerous, with a tuft of long hairs inserted at base.