Plant protease inhibitors are diverse in number and in specificity towards various proteolytic enzymes. This book focuses on the isolation, structure, characterization, physiochemical and physiological properties, synthesis, functions and mechanisms of action of plant protease inhibitors. It deals with their inhibitory effects on insects and pathogens and with their induction and introduction into plants by genetic engineering for plant protection. The induced resistance of insects to the inhibitors and the hazards to friendly insects are also discussed. Further, findings are presented on the cancer chemopreventive properties of the inhibitors at the cellular, tissue and organ levels and in the organism.