Conventional plant breeding can accelerate crop improvement by crossing superior plants with other compatible plants, or randomly induced variants generated by chemicalor radiation-induced mutagenesis. However, its contribution to crop improvement may be limited by a declining genetic base that depends on existing natural allelic variations. Moreover, conventional mutation is time consuming and requires expensive screening of large populations. During the past 20 years, transgenesis has been used for crop improvement. For example, in the USA, more than 90% of cultivated soybeans and corn contain transgenes that confer traits such as resistance to insects or herbicides. Unlike conventional breeding, the production of transgenic plants can overcome natural barriers to breeding, and thereby increase the available genetic variation. Transgenesis, however, has its limitations. Transgenic crops generally carry foreign genes inserted randomly in the genome, and their commercialization is frequently prevented by public concern over health and environmental safety issues. Hence, the needs of an ever-increasing human population call for new and publically acceptable breeding techniques that can rapidly, efficiently, and accurately produce innovative varieties.