The Ras superfamily (>150 human members) encompasses Ras GTPases involved in cell proliferation, Rho GTPases involved in regulating the cytoskeleton, Rab GTPases involved in membrane targeting/fusion and a group of GTPases including Sar1, Arf, Arl and dynamin involved in vesicle budding/fission. These GTPases act as molecular switches and their activities are controlled by a large number of regulatory molecules that affect either GTP loading (guanine nucleotide exchange factors or GEFs) or GTP hydrolysis (GTPase activating proteins or GAPs). In their active state, they interact with a continually increasing, functionally complex array of downstream effectors. Since the last Methods in Enzymology volume on this topic in 2000, the study of Ras Family GTPases has witnessed a plethora of new directions and trends. With regards to the founding member of the Ras superfamily, the study of Ras in oncogenesis has seen the development and application of more advanced model cell culture and animal systems. The discovery of mutationally activated B-Raf in human cancers has injected renewed interest in this classical effector pathway of Ras.