The insecticidal proteins produced by Bacillus thuringiensis (Bt) have provided a uniquely specific, safe, and effective tool for the control of a wide variety of insect pests.  Bt has been used in spray formulations for over 40 years, where it is considered remarkably safe, in large part because specific formulations harm only a narrow range of insect species.  Today, Bt insecticidal protein genes have been incorporated into several major crops where they provide a model for genetic engineering in agriculture. Effective protection of crops from insect pests afforded by insecticidal proteins has had a number of positive impacts on agriculture.  Reduced insect damage improves crop yield, reduces fungal toxins in the food supply, and improves the livelihood of farmers.  Replacement of toxic chemical pesticides with Bt has reduced hazards to the environment and farm workers.  Bt-engineered crops are equally amenable to use in large or small scale farming operations and compatible with other agricultural practices and technologies. Concerns associated with the use of Bt include potential for harm to non-target organisms, development of resistance in populations of target insects, and, for engineered crops, possible ecological consequences of gene flow to non-engineered crops and wild relatives.  These concerns merit continued attention on a case-by-case basis in order to ensure that Bt technologies have the maximum positive impact with a minimum risk on agriculture.  Prudent use of Bt technologies will also be key in maintaining their usefulness for a long period of time.