Expansins are a class of proteins first discovered as mediators of pH-dependent extension of primary cell walls. Structure, classification, activity, and functions of expansins are considered. Because of the lack of enzyme activity and owing to their reversible action on cell wall extensibility, expansins are assumed to disrupt hydrogen bonds between polysaccharides in cell walls strained mechanically by turgor pressure. Expansins are divided in four families: expansins A (EXPA), expansins B (EXPB), expansin-like proteins A, and expansin-like proteins B. Expansins A affect preferentially xyloglucan-rich type-I cell walls characteristic of dicotyledonous plants, whereas expansins B modify type-II cell walls, rich in arabinoxylans and β-glucans, characteristic of grasses. Each plant contains numerous genes coding for expansins. The transcription of these genes changes during plant growth and development and is controlled by phytohormones and environmental conditions. The most documented function of expansins is regulation of cell extension. Nevertheless, a slow retardation of growth during cell differentiation is apparently caused by the loss of cell wall sensitivity to expansins rather than by the decline in expansin activity. Expansins regulate the tip growth of root hairs and cottonseed hairs. The initiation of leaf primordia is related to local expression of expansin genes in the shoot apical meristem. Expansins are also needed for cell wall softening in ripening fruits, in the abscission zone, and in the pistil tissues.