RNA interference (RNAi), one of the most exciting discoveries of the last decade, is also known as Post-Transcriptional Gene Silencing (PTGS). It is a conserved host defense response via double-stranded RNA against both endogenous and foreign nucleic acids, and occurs at the levels of transcription, post-transcription and translation. RNA induced gene silencing has been used, with considerable success as a reverse genetics tool for the study of gene function. This technology has revolutionized the gene expression analysis studies because of the specificity and speed it offers. The small RNA molecules central to RNAi are microRNA (miRNA) and small interfering RNA (siRNA). These molecules bind to other RNAs specifically, such as messenger RNA (mRNA), to regulate their activities.In this book, we will discuss how gene silencing methodology has been successfully applied as therapeutic tool against metabolic disorders, cancer, and infectious diseases. This tool has also been successfully used to improve crops resistance against plant pathogens. In contrast to conventional knock-out mutagenesis techniques, the "gene-silencing" vectors, that allow the study of functions of genes whose complete knock-out would otherwise be lethal to the plant.