The search for proteins which interact with the active GTP-bound form of Ras in order to transmit signals for proliferation, differentiation and oncogenesis has been a long one. Now there are several strong candidates for Ras effectors that include protein kinases, lipid kinases and guanine nucleotide exchange factors. Structural information on how one Ras-binding domain in an effector interacts with Ras.GTP has recently been obtained. Recent data show that transformation by Ras oncoproteins requires the activation of several signal transduction pathways, including those which transmit signals via members of the Rho family of GTPases.