An overview of the ecological impact of transgenic crops, especially Bt cotton, is given in this paper. Crops expressing insecticidal crystal protein (ICP) genes from Bacillus thuringiensis (Bt) were among the first transgenic products approved for commercial use in the USA and several other countries. The cotton farming community in the USA embraced the transgenic technology because of potential benefits such as reducing (1) costs of operation and (2) damage to the environment and ground water supply, due to the repeated use of pesticides. However, concerns about genetically modified crops and foods remain in the USA and elsewhere. The transfer of Bt genes to wild relatives and neighboring crops, impact of Bt on non-target organisms, effects on crop yield and the possibility of insect pest populations developing resistance have all been items of concern. Key aspects of these benefits and concerns are summarized here.