EPA set national emissions standards under section 202 (a) of the Clean Air Act to control greenhouse gas (GHG) emissions from medium- and heavy-duty engines and vehicles, as part of a joint rulemaking with the National Highway Traffic Safety Administration (NHTSA). This joint rulemaking effort was announced by President Obama on May 21, 2010. EPA and NHTSA published a joint Notice of Proposed Rulemaking in the Federal Register on November 30, 2010 (75 FR 74152, November 30, 2010). This rulemaking will significantly reduce GHG emissions from future medium- and heavy-duty vehicles by setting GHG standards that would lead to the introduction of GHG-reducing vehicle and engine technologies. This action follows the U.S. Supreme Court decision in Massachusetts vs. EPA that greenhouse gases are air pollutants under the Clean Air Act, and that EPA therefore has a duty to determine, based solely on scientific considerations, whether that air pollution endangers public health or welfare. EPA has since determined that six greenhouse gases taken in combination endanger the public health and welfare and that the combined emissions from new motor vehicles and motor vehicle engines contribute to the greenhouse gas air pollution that endangers public health and welfare. This rulemaking also follows the Advance Notice of Proposed Rulemaking Regulating Greenhouse Gas Emissions Under the Clean Air Act, (73 FR 44354, July 20, 2008).