Since the first description of RNA interference (RNAi) in animals less than a decade ago, there has been rapid progress towards its use as a therapeutic modality against human diseases. Advances in our understanding of the mechanisms of RNAi and studies of RNAi in vivo indicate that RNAi-based therapies might soon provide a powerful new arsenal against pathogens and diseases for which treatment options are currently limited. Recent findings have highlighted both promise and challenges in using RNAi for therapeutic applications. Design and delivery strategies for RNAi effector molecules must be carefully considered to address safety concerns and to ensure effective, successful treatment of human diseases.