Plant leaves emit a broad spectrum of organic compounds that typically play multiple roles in plant protection. Furthermore, most of these compounds actively participate in tropospheric chemistry. There has been rapid progress in understanding how the emission of volatiles is regulated, mostly focusing on the biochemical controls over compound production. However, physicochemical characteristics such as low volatility or diffusion can also control the emissions and interact with physiological limitations. In particular, nonspecific leaf storage of less volatile compounds smooths the emission responses to fluctuating environmental conditions, and diffusion through stomata leads to conspicuous emission bursts after stomatal opening and modifications of diurnal emission time courses. Because natural conditions always fluctuate, both physiological and physicochemical controls exert a major influence over plant volatile emissions.