RNA interference (RNAi) has been very successfully applied as a gene-silencing technology in both plants and invertebrates, but many practical obstacles need to be overcome before it becomes viable in mammalian systems. Greater specificity and efficiency of RNAi in mammals is being achieved by improving the design and selection of small interfering RNAs (siRNAs), by increasing the efficacy of their delivery to cells and organisms, and by engineering their conditional expression. Genome-wide functional RNAi screens, which are predominantly done in worms and flies, have now begun to revolutionize large-scale loss-of-function studies in mammals.