Plant-generated volatile organic compounds (BVOCs) play key roles in large-scale atmospheric processes and serve the plants as important defense and signal molecules. The main emphasis in quantitative BVOC studies has been on constitutive emissions of isoprene and specific monoterpene species that are present in only certain emitting plant species. However, environmental and biotic stresses can induce emissions of an array of organic compounds in any plant species, whereas the magnitude of emissions induced by given stress depends on stress tolerance, timing, duration and severity (mild versus strong) of the stress. The main view put forward in this review is that quantitative understanding of stress effects is the key for constructing realistic models of both constitutive and induced BVOC emissions.