Plants emit a substantial portion of their fixed carbon back into the atmosphere in the form of volatile organic compounds (VOCs). This process costs plants a large portion of fixed energy that could otherwise be allocated for growth, development and reproduction. However, VOCs have been implicated in a range of ecological functions: as direct defenses against herbivores and pathogens, for the attraction of natural enemies of herbivores, in priming and eliciting defenses, and for the mitigation of oxidative stresses. Although VOCs are chemically highly variable, in this thesis I investigated the biological significances of the C15 sesquiterpene volatiles in plant defense against biotic and abiotic stresses by genetically manipulating nonproducing plants with the respective sesquiterpene biosynthetic genes.