RNA duplexes of 21 - 23 nucleotides (nts), with approximately 2 nt 3' overhangs (called small interfering RNAs or siRNAs), have recently been shown to mediate sequence-specific inhibition of gene expression in mammalian cells via a post-transcriptional gene silencing (PTGS) mechanism termed RNA interference (RNAi). RNAi has been rapidly adopted as a functional genomics tool in a wide range of species, has been adapted to allow for the transient or stable knockdown of gene expression generation in cell lines and animals, and has been developed for high-throughput analysis of gene function in Caenorhabditis elegans. With an increasing list of genes successfully knocked-down by RNAi in mammalian cells and improvements in the delivery of siRNAs to cells, including in vivo delivery to mice, attention is now turning to assessing the potential RNAi has as a gene therapy approach. RNAi is likely to have the greatest impact as a therapeutic tool in two key clinical areas, cancer and infectious disease, but it also has the potential as a therapy for other disorders including some dominant genetic diseases. This review will describe the status of the science behind this novel mechanism and will illustrate the therapeutic potential of RNAi-based technologies, using examples from these critical clinical research areas.