Background: Decades of air quality data from the national ambient air quality monitoring system show that the United States has made significant progress in reducing air pollution levels, but air pollutants continue to harm public health in some places. That system is unable to provide some of the information that users need to better manage health risks from air pollution. For example, the system can miss pollution at smaller scales and in rural areas, and it generally does not monitor air toxics. Emerging lower-cost sensor technologies may help close these gaps. These sensors can be deployed virtually anywhere and can make frequent measurements. However, these sensors can have limitations, such as producing data with variable quality or that are not easily understood or interpreted by users. Key Questions: 1) What are current and emerging lower-cost, outdoor air quality sensor technologies, and what are their performance characteristics? 2) What are current and potential applications of lower-cost, outdoor air-quality sensors? What, if any, are benefits and challenges to developing and using air quality sensors? 3) What policy options may help address challenges to the development and application of air quality sensors?