Microbial Bacillus thuringiensis (Bt) insecticides have been used for over 40 years. In the United States, Bt formulations are primarily applied to control lepidopteran pests on fruit and vegetable crops, to control gypsy moth in forests and to control dipteran pests (mosquitoes and blackflies) that bite humans. A highly selective insecticide with activity conferred primarily by insecticidal crystal proteins (ICPs), Bt is generally not harmful to humans, non-target wildlife or beneficial arthropods. Its selectivity and unique mode of action make it an important alternative to conventional chemical insecticides, and many integrated pest management (IPM) programs for particular fruit and vegetable crops as well as certified organic production include the use of Bt. Agricultural commercialization and adoption of plant-incorporated Bt presents new opportunities to expand the use of Bt ICPs for agricultural pest control, but also raises concerns about the potential for accelerated development of pest resistance to Bt. The relative risks and benefits of microbial and plant-incorporated Bt products are introduced.