Plant viruses cause considerable losses to crops and none of the three current approaches to disease control are likely to give the long-term answer . The new concept of non-conventional protection, in which the expression of a viral or virus-related sequence in the plant genome interferes with the virus infection cycle, holds considerable promise for designing new resistance or protection `genes' . A series of targets in the viral genome is identified and a range of mechanisms for attacking those targets is discussed . There are several current systems which have been proved to give protection, at least to a certain extent, and many systems which are being researched upon for the future . Some of these are described to give a picture of the current situation and of the thinking for the future . The problems of field deployment of the transgenic plants are discussed, especially those associated with the risk to the environment . Various questions which molecular biologists and plant breeders will have to consider include what are desirable characters to have in protection `genes' and how these new `genes' should be deployed .