Several studies have demonstrated the use of plant viruses as vehicles to introduce and express nonviral genes in plants. Many plant viruses multiply to intensely in plants, leading to concomitantly high levels of nonviral gene expression. Plant virus expression vector technology can be improved along several lines. Not all viruses are equally sensitive to sequence manipulations and may be less prone to insert instability. New vectors and new expression strategies are needed to expand the range of plants in which the systems can be used. To increase the carrying capacity or the ability to express more than one nonviral gene, new vectors should be developed.