The Ras and Rho GTPases contribute to the initiation and progression of cancer by subverting the normal regulation of specific intracellular signalling pathways. As a result, Ras and Rho play significant roles in the development of numerous aspects of the malignant phenotype by promoting cell cycle progression, resistance to apoptotic stimuli, neovascularisation and tumour cell motility, invasiveness and metastasis. With these GTPases contributing at so many levels, they are appealing targets for the development of cancer chemotherapeutic agents.