Trees or shrubs often clothed with a scaly indumentum. Leaves alternate or opposite, entire, exstipulate. Flowers usually small, regular, unisexual or bisexual, disposed in axillary clusters, panicles or catkins. Male flowers amentaceous, solitary in the axil of a bract, with a 2- or 4-lobed perianth. Female and hermaphrodite flowers with a free tubular perianth. Stamens 3, 4 or 8, sessile. Fruit superior, 1-celled, 1 -seeded, enclosed in the perianth-tube; seed erect. A small order consisting of four genera and about thirty species, for the greater part natives of the northern hemisphere.