Secondary metabolites play an important role in the defense of plants against herbivorous insects. Attack by herbivorous insects activates a plant’s defense signal pathway, causing it to synthesize a large number of secondary metabolites. These secondary metabolites have a variety of adverse effects on herbivorous insects, including lethality, repellence, appetite suppression and interference with enzyme activity. They can also influence insects’ growth, development, reproduction and symbiotic bacteria, and attract predators of herbivorous insects. This article reviews research on the defensive properties of plant secondary metabolites, and provides a theoretical basis for developing more environmentally-friendly, and sustainable, methods of pest control.