A collection of readily reproducible methods for the design, preparation, and use of RNAs for silencing gene expression in cells and organisms. The techniques range widely and include methods addressing the biochemical aspects of the silencing machinery, RNA silencing in non-mammalian organisms, and the in vivo delivery of siRNAs and silencing vectors. There are also techniques for designing, preparing, and using RNAs to silence gene expression, for fine-tuning regulation by targeting specific isoforms of a given gene, and for the study and use of microRNAs. The protocols follow the successful Methods in Molecular Biology™ series format, each offering step-by-step laboratory instructions, an introduction outlining the principle behind the technique, lists of the necessary equipment and reagents, and tips on troubleshooting and avoiding known pitfalls.