RNA interference (RNAi) is an important pathway to combat virus infections in insects and plants. Hallmarks of antiviral RNAi in these organisms are: (1) an increase in virus replication after inactivation of major actors in the RNAi pathway, (2) production of virus-derived small interfering RNAs (v-siRNAs), and (3) suppression of RNAi by dedicated viral proteins. In this chapter, we will review the mechanism of RNAi in insects, its function as an antiviral immune system, viral small RNA profiles, and viral counterdefense strategies. We will also consider alternative, inducible antiviral immune responses.