RNA interference (RNAi) is a phenomenon that has been observed in many species and causes posttranscriptional gene silencing. RNAi targets specific mRNAs for destruction, causing an active gene to be downregulated and in some cases to be knocked out completely in its expression. RNAi technology begins with the introduction of double-stranded RNA (dsRNA) into cells, which initiates a chain reaction of events that ends in the degradation of mRNA that is homologous to the dsRNA. In mammalian systems small interfering RNA (siRNA) of about 21-25 nucleotides must be used to avoid initiating a systemic response intended for defense against viral invasions. Many different methods and vectors have been employed to produce dsRNA and siRNA, which have had varying levels of success. This review addresses the aspects of emerging RNAi technology as well as different approaches used to introduce dsRNAs and siRNAs into cells. RNAi has recently been shown to have promise as a very powerful tool in cancer research, an d, potentially, therapy. Recently, much research has been focused on the search for new therapies. Some of the implications of RNAi for therapy and future treatments of cancer are, therefore, also evaluated.