The phylogenetically conserved cellular phenomenon of RNA interference (RNAi)-the sequence-specific post-transcriptional silencing of gene expression mediated by small double-stranded RNAs-holds substantial promise for basic research and for drug development. Particularly attractive from a medical standpoint is the juxtaposition of new RNAi methodology with established gene transfer strategies, especially viral vectors for efficient and tissue-specific RNAi delivery to patients. Here, we summarize the latest experimental and clinical advances in RNAibased gene therapy approaches. We briefly portray emerging nonviral strategies for siRNA transfer, before comparing the three viral vectors currently predominantly developed as shRNA delivery vehicles, adenovirus, lentivirus, and adeno-associated virus (AAV). Moreover, we describe the most clinically relevant genetic, acquired or infectious targets being pursued for therapeutic purposes. Specifically, we assess the use of vector-mediated RNAi for treatment of viral processes, solid cancers, lymphoproliferative disorders, and neurodegenerative and ocular diseases. In addition, we highlight further emerging applications, including stem cell therapies and animal transgenesis, as well as discuss some of the potential pitfalls and limitations inherent to the individual approaches. While we predict that eventual schemes will be shaped by our increasing understanding of the complexities of human RNAi biology, as well as by progressive refinements of viral shuttle designs, the potential scientific and medical benefits from a successful marriage of RNAi and gene therapy seem enormous.