Isoprene is a non-methane hydrocarbon that is emitted by certain plant species. This compound affects the chemistry of the troposphere because it is oxidized by the hydroxyl radical and its oxidation products are precursors for the photochemical production of ozone. In 1992, we conducted a study on the controls of isoprene emission from a temperate deciduous forest. We draw upon data from this field experiment, and the literature, to describe the biological, chemical and physical processes that control the synthesis, emission and atmospheric lifetime of isoprene. Isoprene biosynthesis is associated with photosynthesis. Once produced, isoprene molecules diffuse through the stomata and laminar boundary layer of leaves to reach the atmosphere. Then isoprene molecules are transported by turbulence through the plant canopy and into the atmosphere's boundary layer. The ultimate fate of isoprene is controlled primarily by chemical oxidation and deposition to the surface. Emission rates of isoprene from leaves can be predicted by an algorithm that is a function of light energy and leaf temperature. Scaling of isoprene fluxes from the leaf to canopy scale is accomplished by linking the leaf algorithm to a canopy micrometeorological model. Field tests of the scaling model indicate that it can successfully estimate canopy-scale isoprene flux densities, as long as the biomass of isoprene emitting plants is used as a driving variable.