RNA interference (RNAi) is the process of using specific sequences of double-stranded RNA (dsRNA) to knock down the expression level of sequence-homologous genes. Such ability of small interfering RNA (siRNA) in mammalian cells will undoubtedly revolutionize the study of functional genomics, the discovery of drug targets and even the treatment of human diseases. In this review we briefly describe the history of RNAi discovery, the RNAi mechanism and the general guideline for siRNA design as well as various methods for siRNA production and delivery. We also introduce the potential applications of siRNA, inducible siRNA and siRNA library in speeding up basic biomedical research and in acting as potential therapeutic agents for treatment of numerous human diseases.