RNA silencing refers to the process by which the expression of one or more genes is downregulated or completely suppressed by small noncoding RNAs. It also refers to the introduction of an antisense RNA molecule on gene expression. RNA silencing is also defined as the sequence-specific regulation of gene expression triggered by double-stranded RNA (dsRNA). RNA silencing mechanisms are highly conserved in almost all eukaryotes. The most common and well-studied example of this is RNA interference (RNAi), in which endogenously expressed microRNAs (miRNAs) or exogenously derived small interfering RNAs (siRNAs) induce degradation of complementary messenger RNA (mRNA) . Other classes of small RNAs have been identified; including piwi-interacting RNA (piRNA) and its subspecies repeat associated small interfering RNA (rasiRNA). Currently, three main classes of small RNAs have been identified, namely: small interfering RNAs (siRNAs), microRNAs (miRNAs), and piwi-interacting RNAs (piRNAs). Small interfering RNAs (siRNAs) act in the nucleus and cytoplasm and are involved in RNAi. Whereas siRNAs come from long dsRNA precursors derived from a variety of single-stranded RNA (ssRNA) precursors, such as sense and antisense RNA. siRNAs can also originate from hairpin RNAs that arise from inverted repeat regions. siRNAs can also be enzymatically derived from noncoding RNA precursors. MicroRNAs (miRNAs) act in the cytoplasm and mediate mRNA degradation or translation arrest. However, some plant miRNAs have been shown to act directly to promote DNA methylation. MiRNAs originate from hairpin precursors generated by RNaseIII enzymes, namely Drosha and Dicer. miRNA and siRNA form the RNA-induced silencing complex (RISC)... at the center of the article... can interfere with multiple mRNA species, but each with different efficiencies based on the degree of complementarity. Second, miRNAs usually target the 3' noncoding region of RNA transcripts, while most scientists design shRNA constructs for coding regions of mRNAs. The third, and perhaps most complex, miRNA can be transcribed in clusters. Furthermore, these clusters may contain miRNAs that are identical, similar, or distinct from those in other clusters. A cluster could interfere with a large group of targets, producing a large change in phenotype. A second cluster, induced by a distinct signal, could control some of the same targets as well as distinct mRNAs, producing a different phenotype. Added to this is the possibility that some miRNAs can interfere with receptors or transcription factors, which in turn affect multiple pathways and only one, having a system of considerable complexity.