Plant viruses often cause considerable crop damage and significantly reduce yields in the absence of effective protection treatments. In cultivated crop plants the decrease in yield and crop value caused by virus infection is estimated at 60 billion US dollars per year. Virus infected crops are always of inferior quality,. which reduces their market value. Modem biotechnology has significant potential to increase agricultural productivity to meet the demand for food from an increasing world population. Transformation of plants with viral genes has been proven in many cases to produce resistance to the virus from which the genes were derived. The development of transformation techniques broadens the possibility of using engineered virus resistance in plant breeding. There are many destructive virus diseases of crop plants worldwide, and biotechnology may be the fastest and most efficient way to produce resistant cultivars. From the original discovery of coat protein-mediated resistance in 1986 to the introduction in 1995 of squash and papaya resistant to different viruses, this technology has finally reached the market-place. Transgenic virus resistance, like genetic resistance, is of great value for modem agriculture and can create even greater savings for subsistence farmers, who cannot afford other measures of crop protection. Other than transgenic solutions, only breeding for virus resistance and vector resistance offer long term solutions to a virus disease problem; however, this process is lengthy and the resistance genes are not always available in the desired species. Genetic engineering offers an alternative in crops where no effective resistance gene(s) has been identified, or as way of creating durable multiple resistance mechanisms by combining natural and transgenic resistance.