**1. Introduction**

The most recent investigation from the World Health Organization's International Agency for Research on Cancer (WHO/IARC) indicates that the incidence of malignant tumors is rising throughout the world. In 2008, 7.6 million people died of malignant tumors and 64% of them occurred in developing countries(Jemal *et al,* 2011). Particularly, the status on the treatment of tumors is extremely harsh in China since both incidence and mortality of malignant tumors in China is drastically rising in the past two decades (Zhang *et al,* 2008). It is expected that this increasing tendency will continue in China in the next two decades(Xv&Dong,2003). There is no doubt that tumor therapy will be for long term a hot topic in the field of life and medicine in the feature.

The extensive understanding of molecular mechanisms underlying tumor development and progression provides more deep insights that tumor is a genetic disease. It appears that the correction of defective genes would likely be a new approach for the treatment of tumors (Wang *et al,* 2010). RNA interference (RNAi) is a double-stranded RNA-induced gene silencing process, by which RNA can specifically silence complementary mRNA in the cells. Fire *et al*. for the first time discovered this phenomenon in the cells of rod-shaped nematomorpha(Fire *et al,* 1998), for which he was awarded the Nobel Prize in Physiology or Medicine in 2006. Further studies found that RNAi exists in almost all multi-cellular organisms and plays an important role in defense against double-stranded RNA virus(Hannon,2002). Also, RNAi-based tumor therapy in animal models has demonstrated that RNAi is a powerful means for tumor therapy(López-Fraga *et al,* 2009;Davis *et al,* 2010; Koldehoff& Elmaagacli,2009; Maples *et al,* 2009).

This chapter aims to review current progress in RNAi for tumor therapy. The mechanisms underlying RNAi-induced gene silencing pathway were briefly described in the first section. Next, the application of RNAi targeting different tumor-related genes was summarized in detail. Finally, recent achievement of RNAi-based tumor therapy in clinical trials was also reviewed.
