The Rho family of small GTPases act as intracellular molecular switches that transduce signals from extracellular stimuli to the actin cytoskeleton and the nucleus. Recent evidence implicates Rho GTPases in the regulation of neuronal morphogenesis, including migration, polarity, axon growth and guidance, dendrite elaboration and plasticity, and synapse formation. Signalling pathways from membrane receptors to Rho GTPases and from Rho GTPases to the actin cytoskeleton are beginning to be discovered. Mutations in these signalling pathways have been reported in human neurological diseases, which underscores their importance in the development and function of the nervous system.