The microbe Bacillus thuringiensis (Bt) produces crystals that contain insecticidal crystal proteins (ICPs) used to control many major pests. ICPs are degraded by proteases from a variety of sources, including those endogenous to the bacterium, those purified from animals and plants, or those found in insects. Proteases in the bacterium function in protein metabolism during sporulation; in some cases they hydrolyze ICPs. Insect proteases are implicated in Bt toxin specificity, mode of action and insect adaptation to Bt. This review describes the current knowledge of protease interactions with ICPs with special emphasis on the role of proteases in insect resistance to Bt toxins. Arch. Copyright 1999 Wiley-Liss, Inc. This article is a US Government work and, as such, is in the public domain in the United States of America.