Several recent events in mobile sources emissions measurements have caused a major reassessment of the amounts of volatile organic compounds (VOC) and carbon monoxide (CO) in the ambient air attributed to mobile sources. These include the comparison of real-world in-use vehicle emissions with those predicted by mobile emission factor models and the development of cross-road remote sensors for measuring emissions from in-use passing vehicles. The real-world measurements were generally higher than model predictions. A tunnel study in the Southern California Air Quality Study (SCAQS) found two-fold to seven-fold differences. Additional real-world studies, including the remote sensor, roadside I/M inspections and testing on the U.S. Environmental Protection Agency portable dynamometer have shown that 50% of the tailpipe emissions come from only 10% of the vehicles and 80% come from 20% of the vehicles. This paper describes the portable dynamometer study and several tunnel studies.