RNA silencing is described in plants and insects as a defence mechanism against foreign nucleic acids, such as invading viruses. The RNA silencing-based antiviral defence involves the production of virus-derived small interfering RNAs and their association to effector proteins, which together drive the sequence specific inactivation of viruses. The entire process of antiviral defence 'borrows' several plant factors involved in other specialized RNA silencing endogenous pathways. Different viruses use variable strategies to infect different host plants, which render the antiviral RNA silencing a complex phenomenon far to be completely clarified. This chapter reports current advances in understanding the main steps of the plant's RNA-silencing response to viral invasion and discusses some of the key questions still to be answered.