Transgenic crops that are genetically modified to produce insecticidal proteins from the common bacterium Bacillus thuringiensis (Bt) can help to control pests while reducing reliance on insecticide sprays. So far, no insects have evolved resistance in the field to Bt transgenic crops. However, diamondback moth populations have evolved resistance to Bt sprays in the field and many pests have evolved resistance to Bt toxins in the laboratory. To delay resistance, the refuge strategy provides host plants that do not produce Bt toxins, thereby promoting survival of susceptible pests. In Arizona, Bt cotton has been extremely effective in controlling the pink bollworm (Pectinophora gossypiella), a major pest. Despite a surprisingly high frequency of resistance in 1997, resistance did not increase in Arizona field populations of pink bollworm from 1997 to 1999. Nonetheless, pink bollworm and other insects will eventually evolve resistance, so any particular transgenic crop variety is not a permanent solution to pest problems. Instead, transgenic crops can be used in harmony with other tactics as part of integrated pest management. Evaluations of transgenic crops should consider their advantages and disadvantages compared with alternatives. If transgenic crops can greatly reduce use of hazardous insecticides, as achieved in Arizona cotton, great benefits may occur.