RNA interference (RNAi)-based gene therapy has drawn tremendous attention due to its highly specific gene regulation by selective degradation of any target mRNA. There have been multiple reports regarding the development of various cationic materials for efficient siRNA delivery, however, many studies still suffer from the conventional delivery problems such as suboptimal transfection performance, a lack of tissue specificity, and potential cytotoxicity. Despite the huge therapeutic potential of siRNAs, conventional gene carriers have failed to guarantee successful gene silencing in vivo, thus not warranting clinical trials. The relatively short double-stranded structure of siRNAs has resulted in uncompromising delivery formulations, as well as low transfection efficiency, compared with the conventional nucleic acid drugs such as plasmid DNAs. Recent developments in structural siRNA and RNAi nanotechnology have enabled more refined and reliable in vivo gene silencing with multiple advantages over naked siRNAs. This review focuses on recent progress in the development of structural DNA/RNA-based RNAi systems and their potential therapeutic applications. In addition, an extensive list of prior reports on various RNAi systems is provided and categorized by their distinctive molecular characters.