By changing the chemical composition of foliage, the increase in atmospheric CO(2) is fundamentally altering insect herbivory. The responses of folivorous insects to these changes is, however, highly variable. In this review we highlight emerging mechanisms by which increasing CO(2) alters the defense chemistry and signaling of plants. The response of allelochemicals affecting insect performance varies under elevated CO(2), and results suggest this is driven by changes in plant hormones. Increasing CO(2) suppresses the production of jasmonates and ethylene and increases the production of salicylic acid, and these differential responses of plant hormones affect specific secondary chemical pathways. In addition to changes in secondary chemistry, elevated CO(2) decreases rates of water loss from leaves, increases temperature and feeding rates, and alters nutritional content. New insights into the mechanistic responses of secondary chemistry to elevated CO(2) increase our ability to predict the ecological and evolutionary responses of plants attacked by insects.