Due to human activities including fossil fuel burning, deforestation and biomass burning, global atmospheric carbon dioxide (CO2) is expected to double by the end of the century and background tropospheric ozone (O3) is expected to triple within the next 40 years. In addition, under the right conditions, acute and high levels (spikes) of O3 can form. Elevated CO2 induces an increase of carbon assimilation in plants through increased photosynthesis; this is called the "CO2 fertilization effect." Furthermore, the Carbon Nutrient Balance Hypothesis (CNBH) proposed by Bryant and others, suggests that this extra carbon is often incorporated into carbon-based defensive compounds; plant-herbivore interactions often experience important shifts as a result. Elevated tropospheric O3 has a number of negative impacts on plant health and chemistry, which can significantly alter plant interactions with herbivores and vulnerability to disease stress. While elevated CO2 and O3 levels on their own have been shown to have impacts on plant health and plant-herbivore interactions, their combined effects on plants are still not well elucidated. Given that the Earth will experience elevated levels of both CO2 and O3 for the foreseeable future, it is essential to study their combined effects on plants. And since plants are the fundamental basis of all terrestrial systems, a shift in their biochemical makeup will have important impacts on higher trophic levels. Phytoestrogens comprise a diverse class of plant chemicals found throughout the plant kingdom with a variety of known and implied ecological roles, although they receive most of their notoriety due to their impacts on human health. Phytoestrogens are carbon-based compounds, and according to the CNBH, it is likely that their levels will increase under future atmospheric conditions. To date, the effects of elevated CO2 and O3 levels on these plant compounds have not been examined. In addition, the limited data suggest that phytoestrogens are inducible (i.e. increase in response to damage). This research proposal will be the first to examine the effects of elevated CO2 and O3, separately and combined, on constitutive and induced levels of phytoestrogens, and the consequence of these changes on phytoestrogen-based ecological roles, particularly interactions with insect herbivores, pathogens and mutualists.