Ras plays a major role as a molecular switch in many signal transduction pathways which lead to cell growth and differentiation. The GTPase reaction of Ras is of central importance in the function of the switch since it terminates Ras-effector interactions. GTPase-activating proteins (GAPs) accelerate the very slow intrinsic hydrolysis reaction of the GTP-bound Ras by several orders of magnitude and thereby act as presumably negative regulators of Ras action. The GTP hydrolysis of oncogenic mutants of Ras remains unaltered. In this review we discuss recent biochemical and structural findings relating to the mechanism of GAP action, which strengthen the hypothesis that GAP accelerates the actual cleavage step by stabilizing the transition state of the phosphoryl transfer reaction.