The crystals and spores of Bacillus thuringiensis (Bt) have been used for many years as microbially produced insecticides with mixed success. Many of the problems of using Bt as a spray, such as environmental inactivation of the proteins or poor crop coverage, can be circumvented by modern genetic engineering techniques. These can now be used to transfer the genes for the toxic Bt crystal proteins from the bacteria into crop plants and so protect them from attack by economically important insect pests. For many years, the two major obstacles limiting the potential commercial use of transgenic plants expressing these insecticidal Bt proteins were the introduction of Bt genes into important agricultural species and having them expressed at sufficiently high levels to achieve insect control. Many of the technical limitations have now been overcome and the first commercial releases of transgenic insect resistance crops, like cotton are now, or soon will be, in the hands of regulatory bodies. Transgenic seed should hopefully come on the market over the next 4 or 5 years if general approval is given. One of the major considerations that might delay commercialisation is the possibility that insects may become resistant to the Bt proteins expressed in transgenic plants. Considerable research into the deployment of transgenic Bt plants on farms and/or in the production of multiply resistant transgenic plants will still be needed to ensure the effective use of this valuable agricultural resource.