The development of Bt crops is one of the most significant advances in crop protection technology of the past fifty years. Current Bt crops are based on highly specific insecticidal Cry proteins of Bacillus thuringiensis. Foliar Bt insecticides based on the same proteins have been used for more than forty years and have a remarkable safety record, with no known detrimental effects reported on vertebrate or non-target invertebrate populations. Bt cotton and Bt corn have been widely adopted by farmers in the United States, with acreage averaging 40-50% of the area planted with cotton or corn in 2002. Concern has been raised about the safety of Bt crops. However, most evidence shows that these crops, like foliar Bt insecticides, are safe for non-target organisms, especially in comparison to chemical insecticides. Evidence for safety comes from knowledge of Cry protein mode of action as well as from studies of the effects of Cry proteins on non-target organisms tested in the laboratory and under operational growing conditions. These studies indicate that Bt crops, owing to their high degree of safety, can serve as the cornerstone for more environmentally sound integrated pest management programs.