Plants contain a number of volatile organic compounds, including isoprene, mono- and sesquiterpenes, alcohols, aldehydes, ketones, and esters. Individual plant species have unique combinations of these compounds; consequently, the emission pattern for each species is also specific. The factors controlling the emissions were evaluated from the literature and with a simulation model. While unresolved questions remain concerning monoterpene production, storage, and emissions, the literature and model are in general agreement. Environmental factors influence the rate of emission while genetic factors control the pool size and types of chemicals in the plant. The emission of compounds stored in resin ducts of leaves are primarily controlled by temperature, concentration in the resin ducts, and diffusion path from the resin duct to the intercellular air space, and that stomatal conductance plays a minor role.