The discovery of RNA interference (RNAi) for target-specific gene silencing via short interfering RNA (siRNA) has rapidly created a powerful tool for the exploration of pathogenesis of disease. The identification of this remarkable molecular pathway has manifested in the new field of RNAi therapy. In efforts to establish the therapeutic application of RNAi therapy, a major focus has been on target gene-specific siRNA-delivery technology in vivo. In particular, creating a pinpoint delivery system for siRNAs is a priority because such a system would be a key technology for the development of the next generation of drugs, including anticancer therapies. Drug discovery studies and novel treatments based on RNAi are currently targeting a wide range of diseases, including viral infections and cancer. This feature review focuses on recent progress in the nonviral systemic delivery of siRNA in animal models and in clinical trials, as well as on the application of microRNAs in RNAi therapy.