Exo- and endogenous RNA interference (RNAi) is a recently developed post-transcriptional gene silencing system and is regarded as one of the most effective and specific gene silencing techniques with therapeutic potential. siRNA technology is also expected to be an invaluable treatment tool for viral infections, dominant disorders, neurological disorders, and cancers. Novel and increasingly effective therapies are urgently needed as conventional radio/chemotherapy is of limited efficacy in advanced stages of some malignant diseases. In some cancers, it is known that resistance to chemotherapy is mainly attributed to increased expression of anti-apoptotic genes, e.g. members of the bcl-2 family that stabilize the mitochondrial membrane. The siRNA-mediated knockdown of bcl-2 leads to pronounced anti-tumor effects in a pancreatic cancer model, especially in combination with chemotherapy even at otherwise ineffective concentrations. The future success of this approach will depend on the development of effective, specific, and safe delivery systems. In addition to therapeutic RNAi, endogenous RNAi processes may also contribute to cancer development. For example, miRNAs have been shown to suppress target gene expression through binding to the 3'-untranslated regions (3'-UTR) of target mRNAs. miRNAs control many functions in regard to cell viability, including differentiation, proliferation, and apoptosis, especially during cancer progression. More investigation into miRNAs will lead to new diagnostic and therapeutic approaches in the near future.