Genetically engineered crops represent one of the most controversial and rapidly adopted technologies in the history of agriculture. First grown commercially in 1996, transgenic crops covered 135 million hectares (ha) in 25 countries during 2009. To reduce reliance on insecticide sprays, corn and cotton have been genetically engineered to make insecticidal proteins derived from the common bacterium Bacillus thuringiensis (Bt). These Bt toxins kill some devastating insect pests, but unlike broad-spectrum insecticides, they do little or no harm to most other organisms, including people. Many pests have rapidly evolved resistance to insecticides, however, spurring concerns that adaptation by pests could quickly reduce the efficacy of Bt crops and the associated environmental, health, and economic benefits.