RNA interference (RNAi) is an evolutionary ancient mechanism for silencing gene expression. It is found in plants and animals and thus was presumably present in the common ancestor of both. RNAi may have arisen as a host defense mechanism against viruses and other foreign nucleic acids. This role persists: RNAi acts as an innate and adaptive “immune” response against viruses in plants and animals, and in turn viruses have evolved counter measures to abrogate the RNAi response. In a few short years RNAi has become a standard technique in the molecular genetic toolkit and a highly active area of basic research in its own right. This booklet provides an opportunity to peruse studies using RNAi to advance science.