RNA interference (RNAi) has emerged as a novel cellular mechanism regulating gene expression at the post-transcriptional level and as a powerful tool to control gene function experimentally. Recent advances in the biology and application of RNAi include the definition of improved criteria for selecting effective small interfering RNA (siRNA) sequences, and the generation of vectors for the delivery of siRNAs and stable silencing of genes in mammalian cells, tissues and animals. High-throughput screening projects based on RNAi have been initiated to search for genes involved in basic biological processes and in complex pathological conditions such as cancer, autoimmunity and degenerative disorders. This research is helping to identify novel therapeutic targets for a range of diseases and may translate into novel clinical applications for RNAi.