RNAi, a novel strategy of gene-silencing induced by dsRNA, has recently become an important topic of discussion. dsRNAs introduced into a living body are processed by an enzyme known as the Dicer and become fractions of 21-23 nt RNAs (siRNA), after which they form protein complexes known as RNA-induced silencing complexes (RISC). The resulting RISC recognizes cognate mRNAs and cleave them. There are 2 ways to introduce dsRNAs to a living body: direct introduction of synthetic dsRNAs, and introduction of DNA templates into cells, followed by production of dsRNAs by transcription. Now that we can apply RNAi technologies into a living body, RNAi has become a widely used tool due to its great simplicity and high efficiency. However, there are still problems to be solved for more efficient use of RNAi. There are a wide range of applications such as analysis of gene functions and gene therapy. It is suggested that small RNAs, called miRNAs, exist in a living body and bind mRNAs to repress their translations. miRNA plays a great role in gene regulation and control of the differentiation and development of cells.