The overarching goal of the research presented here was to explore how the composition of the future atmosphere will affect the growth and performance of plants. Pursuant to this goal, I determined the single and combined effects of elevated carbon dioxide (CO2), nitrogen dioxide (NO2), ozone (O3) and soil deposited nitrate (NO3-) on seedlings of sugar maple (Acer saccharum), eastern hemlock (Tsuga canadensis), trembling aspen (Populus tremuloides), red oak (Quercus rubra), and the model annual, Arabidopsis thaliana. The chemistry of Earth’s atmosphere is changing, largely due to human activities and these changes include rising concentrations of CO2, NO2, and O3. In order to determine how plants will perform under these likely future atmospheric conditions, we need to understand the mechanisms controlling the entry and elimination of these gases in plant leaves and determine how these gases alter growth, chemistry, and phenology of plants when applied alone and in combinations. I determined the relative importance of physical and chemical processes in controlling the leaf-level fluxes of NO2 and O3 by pairing leaf-level flux measurements with measurements of stomatal conductance, ascorbate concentration, and nitrate reductase activity and using these measurements to build multiple regression models. These models determined that stomatal conductance was the dominant controller of both NO2 and O3 fluxes, explaining 84 and 56% of the variance in NO2 and O3 fluxes, respectively. The addition of ascorbate concentration was particularly useful in the model for O3 fluxes where it explained an additional 10% of the variance. Based on the findings of the modeling study, I predicted that any treatment causing a change in stomatal conductance would likely impact the magnitude of the plant responses to NO2 and O3. From 2004-2007, I conducted field experiments using open-top chambers to expose plants to combinations ofelevatedCO2, NO2,O3, and soil NO3-. The three most important findings from these experiments were: 1)The combination of elevated CO2, NO2, and O3 rarely resulted in a change in plant biomass even if the treatments individually did alter biomass, 2) Even when elevated CO2 did not increase overall biomass, it did alter leaf chemistry and structure by decreasing specific leaf area and % leaf nitrogen while increasing leaf C:N, and 3) Under elevated CO2, elevated O3 significantly delayed the production of flowers and pods in addition to decreasing the overall reproductive output of the model annual, Arabidopsis thaliana. These findings suggest that current models predicting an increase in tree seedling growth under elevated CO2 may be overestimating the potential biomass production because they do not account for the effects of elevated NO2 and 03. These results also suggest that changes in phenology and leaf structure and chemistry may be greater than changes in overall plant biomass and deserve greater attention from the scientific community.