Genetically modified organisms (GMOs) are being released into the environment on a massive scale. By 2000 there were 44 million hectares of transgenic soyabeans, maize, cotton, oilseed rape and potatoes grown worldwide - equivalent to an area twice that of the United Kingdom. Transgenic micro-organisms, trees and fish are also being released into the environment. Has sufficient attention been paid to the environmental costs? Genetic engineering could make a valuable contribution within agriculture, although the initial promise of more abundant food, produced in an environmentally friendly manner, is not being fulfilled. Here, Stephen Nottingham argues that the precautionary principle should be applied far more extensively when GMOs are released into the environment.