Dioecious,mostly deciduous,trees and shrubs,rarely prostate subshrubs. Leaves alternate, simple, usually stipulate, entire or variously toothed or lobed. Flowers in spicate or racemose catkin,erect or pendulous, each flower subtended by a bract or catkin scale; perianth cupulate (females of Populus) or reduced to 1 or more glands (Salix), rarely absent. Stamens 2 to many, filaments free or connate (partially or completely). Ovary superior, uni or bilocular, parietal placentation, usually with many ovules. Fruit a capsule, seeds many, small, with long silky hairs.