Plants not only exchange inorganic C with the atmosphere, but also organic C in the form of a broad range of biogenic volatile organic compounds (BVOCs). The magnitude of the global BVOC flux is small compared to the global photosynthetic CO2 flux: ~ 2 Pg of C contained in annual BVOC emissions (including CH4 and CO) compared to global gross primary productivity, which is ~ 120 Pg of CO2. However, the BVOC flux is critical to understanding chemical reactions that occur in the atmosphere, especially those that produce important oxidant compounds, such as ozone, those that determine the oxidation rate of important greenhouse gases, such as methane, and those that determine the production of organic aerosol particles, which affect the earth’s radiation budget. Scientists did not recognize that the emission of compounds from vegetation could have such an important impact on atmospheric chemistry until the mid 1960s when researchers such as Rei Rasmussen and Fritz Went began collecting air samples in remote locations, far from the influences of urban pollution. This effort to describe the volatile chemical “fingerprints” of natural ecosystems revealed the presence of a large outward flux of reactive compounds that had previously been unidentified. For example, as reflected in the quote above, careful analysis using gas chromatography revealed the presence of reactive isoprene and other terpenes in an oak forest, which would have been undiscovered if left to detection by the human nose alone. Once these observations started to accumulate, it became clear that biogenic sources of reactive compounds were even more important than anthropogenic sources in their potential to catalyze oxidative photochemistry and, in many cases, control the overall oxidative capacity of the troposphere.