The spread of commercial growing of transgenic crops has been very rapid, with such crops being grown on 44.2 million ha in 13 countries in 2000. Most currently grown transgenic plants are either herbicide or insect resistant, but research into other possibilities is intense. There are potentially significant environmental effects of this technology, as well as sharply conflicting claims about its overall impact. Among perceived risks, this review concentrates on the possibility of damaging ecosystem services (biological control, pollination, decomposition and soil fertility maintenance) and effects on biodiversity. Among the benefits, lower environmental contamination by fewer pesticide applications, less soil erosion due to less frequent agricultural operations and lower pressure on land resources due to increased yield, are listed. Both potential risks and benefits need careful documentation and validation for an appropriate assessment of the total ecological effects of this technology.