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, Rho GTPases have continued to receive a huge amount of attention. The human genome sequence has revealed the full extent of the Rho GEF and Rho GAP families (over 80 members for each) and the challenge of identifying the molecular interactions and cellular pathways influenced by each of these regulators is a daunting prospect. This new volume, Regulators and Effectors of Small GTPases: Rho Family, describes some of the methods currently being used to examine Rho family GTPase regulation at the biochemical and cellular level.