Genetic engineering of virus resistance in plants may be conferred by transgenes based on sequences from the viral genome. In many instances the underlying mechanism involves the transgenically expressed proteins. However there are other examples in which the mechanism is based on RNA. It appears that this mechanism is related to post transcriptional gene silencing in transgenic plants. This gene silencing is likely to involve antisense RNA produced by the action of a host-encoded RNA dependent RNA polymerase. The natural role of this mechanism is as a genetic immune system conferring protection against viruses. There may also be a genomic role of the process reflected in RNA directed methylation of transgenes. Further understanding of this mechanism has obvious implications for virus resistance in plants. In addition the gene silencing can be used as a component of a new technology with application in functional genomics.