Expansins are extracellular proteins which are able to loosen cell walls thus making possible the extension and stress relaxation of plant walls. Expansins are classified on the basis of sequence similarities and substrate specificity into two subfamilies of a- and b-expansins. The expansins? mode of action is based on the weakening of hydrogen bonds between cellulose microfibrils and on interacting matrix polymers, especially hemicelluloses. However, they do not reveal any hydrolytic or proteolytic activity. The activity of expansins is decisive for the control of plant cells? shape, and thus these proteins are thought to be an important component of biochemical machineries controlling cell growth and differentiation, as well as plant morphogenesis and development. This article reviews the advances made since the discovery of expansins and describes their characteristic features, including protein structure and biochemical mode of action, molecular organisation of expansin-coding genes, and biological functions of these unusual proteins.