The actin cytoskeleton is involved in numerous cellular functions such as cell motility, mitogenesis, morphology, muscle contraction, cytokinesis, and establishment of cell polarity. The members of the Rho subfamily of small GTP-binding proteins emerge as key regulators of cytokeleton organization. Rho, Rac, and CDC42 are implicated in the regulation of actin microfilament organization of different cell structures, such as stress fibers linked to focal adhesions and membrane ruffles induced by extracellular stimuli. Rho proteins also regulate the activity of several enzymes involved in the formation of phospholipid derivatives, which could mediate their effect on the cytoskeleton. The activity of Rho proteins is regulated by many nucleotide exchange factors and GTPase-activating proteins, some of which are oncogene products, and other disease-associated proteins. The potential role of these small GTP-binding proteins in carcinogenesis is suggested by the actin reorganization seen in transforming cells and by the need for functional Rho proteins in Ras mitogenic activation.