Since the first report of RNA interference (RNAi) less than a decade ago, this type of molecular interventionhas been introduced to repress gene expression in vitro and also for in vivo studies in mammals. Understandingthe mechanisms of action of synthetic small interfering RNAs (siRNAs) underlies use as therapeutic agents inthe areas of cancer and viral infection. Recent studies have also promoted different theories about cell-specifictargeting of siRNAs. Design and delivery strategies for successful treatment of human diseases are becomingmoreestablished and relationships between miRNA and RNAi pathways have been revealed as virus-host cellinteractions. Although both are well conserved in plants, invertebrates and mammals, there is also variabilityanda more complete understanding of differences will be needed for optimal application. RNA interference (RNAi) israpid, cheap and selective in complex biological systems and has created new insight sin fields of cancer research,genetic disorders, virology and drug design. Our knowledge about the role of miRNAs and siRNAs pathwaysin virus-host cell interactions in virus infected cells is incomplete. There are different viral diseases but fewantiviral drugs are available. For example, acyclovir for herpes viruses, alpha-interferon for hepatitis C and Bviruses and anti-retroviral for HIV are accessible. Also cancer is obviously an important target for siRNA-basedtherapies, but the main problem in cancer therapy is targeting metastatic cells which spread from the originaltumor. There are also other possible reservations and problems that might delay or even hinder siRNA-basedtherapies for the treatment of certain conditions; however, this remains the most promising approach for a widerange of diseases. Clearly, more studies must be done to allow efficient delivery and better understanding ofunwanted side effects of siRNA-based therapies. In this review miRNA and RNAi biology, experimental design,anti-viral and anti-cancer effects are discussed.