RNA interference (RNAi) represents one of the most promising new frontiers in drug discovery. Breakthroughs in understanding RNA’s extensive natural role in essential cellular processes have opened up the potential for a whole new class of drugs based on RNAi. Harnessing the natural process of RNAi, short, double-stranded RNA molecules are able to inhibit expression of genes in a sequence-specific manner. By targeting disease-causing genes, RNAi drugs have the potential to be more selective than traditional drugs, and thus more effective as well as less toxic. Over the past few years, important strides have been made in translating the promise of RNAi into therapies for human disease. In contrast to the extensive lead optimization steps typically required in small-molecule and protein drug discovery, RNAi drug candidates can be identified using bioinformatics to select sequences complementary to the target mRNA. The process of selecting an RNAi-based drug candidate may simply involve the synthesis and testing of a relatively small number of short double strands (duplexes) of RNA, incorporating chemical modifications that confer stability and direct these RNA duplexes to the appropriate tissues and cells, and/or formulating these RNA duplexes with appropriate delivery agents to achieve the same goals. As advances in RNAi therapeutics continue, the decades to come should bring a potent new class of drugs based on RNAi.