Genetic engineering allows the genetic modification of a plant by adding new traits through recombinant DNA technology. Genetic engineering makes possible the incorporation of a single gene or a combination of genes into the plant genome in order to confer traits such as disease resistance, herbicide tolerance, tolerance to salt, drought and cold, increased productivity, and increased nutritional value. Genetic engineering offers great potential to complement conventional approaches for the control of plant diseases, and overcome some of their limitations. The chapter focuses on the use of genetic engineering to develop agronomically important crops that are resistant to fungi, bacteria, nematodes, and viruses. It describes several examples of engineered resistance against these phytopathogenic agents with a special emphasis on resistance evaluation carried out under field conditions. The development of genetically modified plants that are resistant to diseases caused by fungi, bacteria, and nematodes has lagged behind that of virus-resistant crop plants.