The new standards and related provisions contained in this final rule will result in significant progress throughout the country in protecting public health and the environment. In this action, EPA is adopting a new emission standard and related provisions for diesel heavy-duty engines (HDEs) intended for highway operation, beginning with the 2004 model year. The new standard represents a large reduction (approximately 50 percent) in emission of oxides of nitrogen (NO<INF>x</INF>), as well as reductions in hydrocarbons (HC) from diesel trucks and buses. The reduction in NO<INF>x</INF> will also result in significant reductions in secondary nitrate particulate matter (PM) in areas where levels of nitrate PM are high. For diesel HDEs, EPA is also finalizing changes to the existing averaging, banking, and trading program that provide additional flexibility for manufacturers in complying with the stringent new standards. EPA is also adopting several provisions to increase the durability of emission controls, help ensure proper levels of maintenance, and prevent tampering, including during engine rebuilding. The resulting emission reductions will translate into significant, long-term improvements in air quality in many areas of the U.S. This will provide much-needed assistance to states and regions facing ozone and particulate air quality problems that are causing a range of adverse health effects for their citizens, especially in terms of respiratory impairment and related illnesses. Although EPA proposed new standards and related averaging, banking, and trading provisions for otto-cycle HDEs (e.g., gasoline-fueled engines), EPA is not taking final action for that category of engines at this time. EPA received several comments urging the Agency to adopt more stringent control measures for these engines than those proposed in the NPRM (June 27, 1996). EPA continues to evaluate the comments received regarding otto-cycle engines and plans to issue a Supplemental Notice of