RNA interference (RNAi) is a post-transcriptional, highly conserved process in eukaryotes that leads to specific gene silencing through degradation of the target mRNA. This mechanism is mediated by double-stranded RNA (dsRNA) that is homologous in sequence to the silenced gene. The dsRNA is processed into small interfering RNA (siRNA) by an enzyme called Dicer, and the siRNAs are then incorporated into a multi-component RNA- -induced silencing complex, which finds and cleaves the target mRNA. In plants and worms, amplification of the silencing signal and cell-to-cell RNAi spreading is observed. The proposed biological roles of RNAi include resistance to viruses, transposons (mainly in plants), and the silencing and regulation of gene expression, particularly during development. In developmental gene control, specific small RNAs (micro RNA and small temporal RNA) are involved, which are processed in the same way as dsRNAs but act at the level of translation. RNAi technology has become a powerful tool in functional genomic analyses and may prove to be a useful method to develop highly specific gene-silencing therapeutics against viral infections and cancer in the future.