The oxides of nitrogen (NOx) emission levels from heavy-duty engines used in vehicles over 8,500 pounds (lbs) gross vehicle weight (GVW) will drop from the current level of 5.0 grams per brake horsepower-hour (g/bhp-hr) to 4.0 g/bhp-hr in 1998. The SOP proposes that engine manufacturers meet a combined standard of 2.4 g/bhp-hr for nonmethane hydrocarbon (NMHC) and NOx emissions by 2004. To reach these low NOx levels and keep particulate matter (PM) emissions at the current levels (0.1 g/bhp-hr for trucks, 0.05 g/bhp-hr for urban buses) or lower, manufacturers will look to combinations of reoptimized combustion chambers, fuel systems, air handling systems, electronic controls and aftertreatment. While manufacturers suggest that the SOP goals of 2.4 g/bhp-hr NOx plus NMHC at current PM levels will not be easy to meet, they agree that these goals are possible to meet by 2004. The methods that they might use to reach the SOP goals are the content of this report. Descriptions of technologies and costs of technologies to meet the proposed 2.4 g/bhp-hr NOx plus NMHC standards were obtained through candid conversations with heavy-duty engine manufacturers, equipment manufacturers, manufacturers associations, research organizations, and various publications. We used this information to present a coherent set of likely technologies for meeting these future standards. When information was not provided or only partially provided, engineering and economic judgement was used to provide additional details.