Despite great progress in reducing emissions from new cars, air quality problems persist in many urban areas. This is in part due to two ”loopholes” in current vehicle emissions regulations: new cars are not tested under representative driving conditions, and in-use emission requirements are not adequately enforced. Researchers at LBNL and the University of Michigan have examined an extensive database of the in-use vehicle emissions to learn more about real-world emissions. Our analysis suggests that a surprisingly large fraction of relatively new cars have malfunctioning emission controls, and that some vehicle models perform significantly worse than others. The results have important implications for both federal and state vehicle emission policies.