Human activities such as the intensive use of fossil fuel have been imposing significant perturbations to the atmosphere and air quality on the global scale. Tropospheric ozone is important as a greenhouse gas, as a precursor for OH, the ultimate cleansing agent in the atmosphere, and as a surface air pollutant toxic to humans and vegetation. Global change in the coming decades, including climate change and changes in anthropogenic emissions can have potentially large effects on tropospheric ozone, on hemispheric transport of air pollution, and on air quality. This book studies these effects with a focus on ozone air quality in the United States. The current generation of global atmospheric chemistry models are intercompared and the factors controlling global ozone budgets are explored. It is demonstrated that the future climate change can degrade the U.S. ozone air quality by affecting the air pollution meteorology such as temperature and ventilation. The changes in anthropogenic emissions outside of the United States impose another challenge by enhancing the hemispheric transport of air pollution and hence background ozone.