Natural gas has many advantages as a fuel compared to coal and oil. It is abundant and can often be produced at low cost. It has significantly lower combustion emissions of numerous species including greenhouse gases, criteria air pollutants (e.g., carbon monoxide, ozone, sulfur dioxide), heavy metals, and acid-forming species (Moore et al. 2014). And with sufficient transmission and distribution infrastructure in the United States and other countries, it can be burned flexibly in sectors across the economy—power generation, industry, homes, and businesses. In electricity generation the advantage of natural gas is clear. Natural gas turbines are pinnacles of engineering achievement: they consume fuel very efficiently and are capable of rapid adjustment in response to changes in power demand and supply. Its advantages lead some to argue that natural gas could have long-term use in renewable-heavy power grids of the future. Recent experience in the United States is illustrative: the share of US electricity generated using gas rose from 18.8 percent in 2005 to 27.7 percent in 2013, mainly displacing coal (EIA 2015) and contributing to a 15 percent reduction in carbon dioxide (CO2) emissions from power generation (EPA 2015a). However, some problems remain with the use of natural gas. First, its combustion releases large quantities of CO2 relative to zero-carbon electricity sources such as wind, solar, hydroelectric, and nuclear. Second, the production and processing of natural gas are not without air quality impacts. These impacts are associated with both the fuel consumption required along natural gas supply chain operations and the release of permitted or unintentional emissions of methane (the main constituent of natural gas) and other components of natural gas. We briefly explain the mechanisms and risks of fugitive emissions, and then review recent assessments of such emissions at the national and local levels. Subsequent sections set out the implications of fugitive emissions for both local air quality and overall climate quality. We conclude with a summary of challenges in regulation and funding to protect air quality and maximize the potential benefits of natural gas as a reliable and abundant source of energy.