Symbiont-mediated RNA interference (SMR) is an alternative approach for delivery of interfering RNA to insects compared to uptake by ingestion or manual injection. To deploy SMR, culturable gut symbiotic bacteria are genetically manipulated to produce interfering RNA, usually double-stranded RNA, to knockdown expression of target genes in the host insect or its pathogens. The manipulated bacteria can then be used to populate the insect gut where they can continuously synthesise this interfering RNA. Using SMR to elicit RNAi can avoid issues concerning the instability of ingested interfering RNA during its transit from the mouthparts to the gut and the trauma associated with injection. Moreover, due to the continuous synthesis of interfering RNA in the gut, SMR can elicit long-term knockdown of target gene expression. This chapter describes SMR in detail and its application in representative species belonging to four insect orders, together with prospects for future developments and applications of the technology.