Publisher Summary This chapter discusses the secondary metabolites in plant-insect interactions. It has long been accepted that plant-insect interactions involve the evolution of defensive secondary metabolic pathways in plants to which insects can adapt. New theories are being proposed to account for the wide diversity of such interactions, particularly where production of plant secondary metabolites is inducible. This chapter briefly describes the more widely known mechanisms by which secondary metabolites act, such as direct toxicity, antifeedancy and sequestration. Key areas covered in greater depth involve the volatile secondary metabolites produced by plants, both inherently and in response to the stress of insect feeding and colonization. Interactions described include the location of host plants by insects and the corresponding avoidance of unsuitable host plants. The roles of volatile plant compounds in the attraction of predators and parasitoids to plants already hosting insect populations, and in the synergism of insect pheromone activity, are discussed.