Heavy-duty engines have been subject to emission standards for particulate matter (PM), hydrocarbon (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) for nearly half a century; however, current data suggest that the existing standards do not ensure full in-use emission control. In particular, in-use engine NOx emission levels from heavy-duty vehicles can be significantly higher than implied by their certified values under certain conditions. NOx emissions are major precursors of ozone and significant contributors to secondary PM2.5 formation. Ozone and ambient PM2.5 concentrations continue to be a nationwide health and air quality issue. Reducing NOx emissions from on-highway heavy-duty trucks and buses is an important component of improving air quality nationwide and reducing public health and welfare effects associated with these pollutants, especially for vulnerable populations and in highly impacted regions. This action will evaluate data on current NOx emissions from heavy-duty vehicles and engines, and options available to improve control of all criteria pollutant emissions, to inform a proposal for revised emissions standards.