Plants show a broad diversity of defense strategies against a wide range of pests. Both constitutively and inducibly in response to biotic stresses, plants release volatile organic compounds including a wide-range dynamic chemical array of terpenoids (terpenes). Terpenes in terrestrial environments are known to function as airborne signals to communicate with other land-living organisms, e.g., insects, bacteria and plants. These chemical compounds, in organisms ranging from algae to higher plants, are biosynthesized via two biosynthetic pathways: the mevalonate pathway and the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway connecting to a large diversity of synthetic and modification enzymes including terpene synthases and cytochrome P450 monooxygenases. Moreover, in terrestrial ecosystems, since the blends of terpenes may vary with the attacking herbivore, various components and crosstalk between the involved signaling pathways are thought to be responsible for the characteristic terpene blend. These specific blends of terpenes provide the majority of constitutive and induced chemical defense systems and serve for attracting carnivorous natural enemies of the herbivores, thus resulting in the removal of herbivorous arthropods from a wide array of plant species. Implications for the nature, biosynthesis and regulation of terpenes in both angiosperms and gymnosperms are discussed.