The recent discovery of RNA interference (RNAi) has revolutionised biological research and now holds promise as a potential therapy for human diseases. Currently untreatable neurological diseases are especially attractive targets. Scientists have already succeeded in using RNAi to suppress dominant disease genes in vitro; in some cases, this suppression has been allele-specific, silencing the disease-causing allele while maintaining expression of the normal allele. The challenge now is to bring this powerful technology in vivo to animal models to suppress disease genes and correct disease phenotypes. In the confrontation of this challenge, research should benefit from recent advances in viral and non-viral delivery of therapy to the brain.