The introduction of novel genes into plants by genetic transformation holds great promise for plant breeding, and many crop species have been rendered virus-resistant by expression of viral sequences. However, it is essential to also evaluate the potential risks associated with this new technology. Among the types of genetically modified plants that could represent potential ecological risks, ones expressing viral sequences pose questions of particular interest. In this volume special attention is given to recombination in plants expressing sequences of RNA or DNA viruses, heterologous encapsidation or other forms of complementation in plants expressing coat protein genes, potential deleterious effects of satellite RNAs associated with cucumber mosaic virus, and sexual transmission of virus resistance genes to potentially weedy relatives.