The breakthrough of RNA interference (RNAi) and related small-RNA mediated gene silencing pathways in eukaryotic cells has significantly given a new understanding of the regulation of gene expression and function.Studies have uncovered the hidden layers of regulation of gene expression, in which many families of small RNAs which were previously unidentified mediate gene silencing.Although RNAi and related gene-silencing pathways regulate the expression of genes by initiating the production of small RNAs with sequences complementary to portions of the transcripts that they regulate.However in different organisms,the RNAi pathways consists of different proteins and mechanisms, but they operate by strikingly convergent strategies.In this book, we describe the role of diverse small-RNA silencing regulatory molecules and proteins, their different pathways in the regulation of gene expression,and we discuss how the different nuts and bolts of gene silencing machineries, their different molecular mechanisms are involved in performing the common task of gene regulation and their promises in theurapetics.