Publisher Summary Reversible sol–gel transformations have been observed in various cells for many years. This chapter presents evidence indicating that protoplasmic gel structures are potentially contractile. The gelational process thus seems to represent a mechanism, which enables the cell to perform mechanical work. The chapter reviews the evidence with special reference to amoeboid movement and the furrowing movements (cytokinesis) in animal cells. The development of contractility in an essentially fluid system, such as protoplasm would seem to presuppose the formation of some kind of gel structure. Gelation represents the formation of a three-dimensional network from fibrillar units present in the system. Thus, the contraction of a gel structure represents a rapid sort of syneresis, whereby the protein components of the colloidal network undergo forcible folding without relinquishing their intermolecular linkages. Subsequent to folding, moreover, when the extended units have assumed a more globular form, the system reverts to the sol condition, merely by the loosening of the intermolecular bonds. The effective contraction of a gel structure necessarily seems to demand that the fluid component of the system, which initially occupies the interstices of the colloid network, be forced forth from the interstices, thus allowing for a fairly drastic shrinkage of the residual framework of the gel.