Abstract: Plant protease inhibitors are considered to be protective agents in plants against herbivorous insects. It was shown that damage to the leaves of certain plant species by insect feeding induced the synthesis of protease inhibitors. Plants can now be genetically manipulated with protease inhibitor genes to express the inhibitor proteins in relatively high level resistant to insect pests. Studies on interactions of protease inhibitors with insect digestive enzymes may further elucidate the physiological role in natural plant defense and the potential usefulness in resistant crop breeding against insect pests of agriculture.