The United States and China, the top two consumers of energy in the world, are also the top two emitters of many pollutants that have local, regional, and global effects. Although U.S. cities continue to face serious air pollution, many have greatly reorganized their energy use and improved their air quality. Nevertheless, air pollution continues to impose major costs—both human and economic. This new report summarizes current trends in addressing urban air pollution, profiles two U.S. and two Chinese cities, and recommends actions to enable both countries to improve urban air quality. The bilateral study committee also concludes that China can learn some specific lessons from the successes and failures of U.S. efforts. The report was conducted and produced by a committee of experts formed by the National Research Council and National Academy of Engineering in cooperation with the Chinese Academy of Sciences and Chinese Academy of Engineering. The overarching conclusion is that both countries must improve energy efficiency in power generation, transportation, and other sectors.