Abstract Until semi-solid metal forming has been introduced into industrial application most of the structural parts achieved their final form either in fully liquid or in fully solid state. The difficulties of forming in the temperature range between the solidus and the liquidus are ascribed to the formation of a solid skeleton of dendrites when the solid volume fraction achieves 20 per cent. This semi-solid material with a dendritic skeleton cannot be homogeneously deformed, since separation of the liquid phase and cracking may occur. On the other hand, by vigorous agitation of the cooling alloy the skeleton formation from dendrites is inhibited, since the solid phase grows in the form of globules instead of dendrites. Such a slurry (or mush) already can be homogeneously deformed. The deformation behaviour of semi-solid alloys, however, significantly differs from the deformation behaviour of both solid and liquid materials. These specific features of the deformation behaviour of the semi-solid materials are presented in this contribution.