It is well documented that exposure to ambient air pollution at concentrations typically found in U.S. cities causes significant health effects. Reducing exposure to air pollution is a large, long-term goal for the environmental health community. In this talk, I will address three questions: 1) How should we prioritize emission reduction efforts? 2) Can urban planning help reduce exposure to air pollution? 3) Are there correlations between exposure to air pollution and demographic attributes such as ethnicity and income? I use three case studies to address these three questions. First, tracer gas experiments show that children commuting in a school bus are exposed to emissions from the bus they're riding on. These experiments highlight why air quality management should consider exposures rather than only emissions. Second, infill development - that is, encouraging urban growth to increase population density rather than sprawling to ”green fields” - can reduce transportation demand and potentially transportation emissions. Yet, it also increases the fraction of emissions that are inhaled because people are in closer proximity to the emissions. Thus, there is the potential for infill to reduce emissions but increase exposures. I use a simplified, hypothetical urban area to explore whether infill development can reduce population exposure to vehicle emissions. Finally, I have developed a mobility-based exposure assessment for California's South Coast Air Basin. I track individuals' inhalation of air pollution as they move around the air basin during the day, interacting with spatially- and temporally-varying ambient concentrations. I investigate whether mobility is important to determining exposure to air pollution, and for whom. My results also quantify the extent to which specific subpopulations are more or less exposed to air pollution.