RNA interference (RNAi) is a conserved process in which a double-stranded RNA (dsRNA) induces sequence-specific gene silencing. Recent developments in the use of the 21-nt small interfering RNA (siRNA) have allowed the specific degradation of mRNA without induction of nonspecific effects in mammalian cells. RNAi provides a method for knocking down genes of interest and a powerful tool for studies on gene functions in various organisms. Although many vector-based siRNA expression systems have been developed for production of siRNAs in mammalian cells, many technical issues for an effective production of siRNAs still need to be resolved. In this chapter, we describe methods for construction of genetically stable and highly active siRNA expression systems and also mention some strategies to overcome serious technical problems.