Since its discovery in 1998 RNA interference (RNAi), a potent and highly selective gene silencing mechanism, has revolutionized the field of biological science. The ability of RNAi to specifically down-regulate the expression of any cellular protein has had a profound impact on the study of gene function in vitro. This property of RNAi also holds great promise for in vivo functional genomics and interventions against a wide spectrum of diseases, especially those with "undruggable" therapeutic targets. Despite the enormous potential of RNAi for medicine, development of in vivo applications has met with significant problems, particularly in terms of delivery. For effective gene silencing to occur, silencing RNA must reach the cytoplasm of the target cell. Consequently, various strategies using chemically modified siRNA, liposomes, nanoparticles and viral vectors are being developed to deliver silencing RNA. These approaches, however, can be expensive and in many cases they lack target cell specificity or clinical compatibility. Recently, we have shown that RNAi can be activated in vitro and in vivo by non-pathogenic bacteria engineered to manufacture Biotechnology and Genetic Engineering Reviews -Vol. 25, 113-128 (2008) * To whom correspondence may be addressed (cli@bidmc.harvard.edu) Abbreviations: APC, adenatomous polyposis coli; dsRNA, double-stranded RNA; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GFP, green fluorescent protein; mRNA, messenger RNA; PKR, protein kinase R; RISC, RNA-induced silencing complex; RNA, ribonucleic acid; RNAi, RNA interference; miRNA, micro RNA; MOI, multiplicity of infection; RIG-I, retinoic acid inducible gene I; shRNA, short hairpin RNA; siRNA, short interfering RNA; SNALP, stabilized nucleic acid lipid particle; tkRNAi, TransKingdom RNA interference; TLR, toll-like receptor; TRIP, TransKingdom RNA interference plasmid.