Monoterpenes resulting from the combination of two and sesquiterpenes from three branched, unsaturated C5-units (isoprene) represent a large class of natural products with a wide range of biological activities. They include unsaturated hydrocarbons and their oxidation products as alcohol, aldehydes ketones, and rarely ethers. They form acyclic, mono-, bi-, or tricyclic, usually lipophilic structures. Volatile compounds are constituents of essential oils that are accumulated by numerous plants in special structures in the tissue. Besides the conspicuous aromatic properties, a wide range of biological activities have been documented allowing a wide field of applications. In an ecological context, mono- and sesquiterpenes play an important role in the relations between organisms, e.g., as attractants of pollinators or deterrents of herbivores. On a large scale, monoterpene emissions from vegetation in nature can have ecosystem-wide influences. Further investigation and documentation of this high biodiversity and its sustainable use remains a promising task. This requires the further development of analytical and production techniques and the exact definition and characterization of the plant sources.