RNA interference (RNAi) is a newly discovered mechanism of the post-transcriptional regulation of gene expression. This phenomena is induced by coded in genome micro RNA (miRNA) or by synthetic, short interfering RNA (siRNA). Both types of RNA molecules after binding to RISC protein complex, exhibiting nucleolytic activity, operate as guides which recognize the complementary sequence in target mRNA. Formation of the siRNA/RISC complex with mRNA results in hydrolysis of the mRNA, and in consequence, in degradation of the translation template. Binding of the miRNA-loaded RISC to mRNA inhibits formation of the translational complex. Discovery of the RNA interference was a breakthrough for development of the functional genomics as well as kindled a new hope for revitalization of the therapeutic use of nucleic acids. At present the siRNA duplexes are widely used as universal, sequence specific inhibitors of gene expression for research purposes. In numerous biotechnological companies intensive studies are focused on introduction of siRNA molecules into therapies of diverse diseases. The most advanced (clinical trials phase II) are the studies on the siRNA directed towards mRNA of VEGF or of its receptor as an approach for treatment of age-related macular degeneration (AMD).