Rho family GTPases have a range of signalling functions. In particular, they play critical roles in regulation of the actin cytoskeleton. The best-known members of the family are Rho, Rac and Cdc42; however, at least 23 Rho-related proteins are present in humans. Mathew Wherlock and Harry Mellor survey the known members of the Rho family, providing Rho phylogenetic trees for six species and detailing the members of the various Rho subfamilies.