Medium- and heavy-duty vehicles (MHDVs) are a significant contributor to energy consumption and greenhouse gas (GHG) emissions in the transportation sector, accounting for approximately 22 percent of U.S. transportation energy consumption.¹ Unlike the light-duty vehicle sector, which has long been subject to corporate average fuel economy standards, the fuel consumption of MHDVs has only recently begun to be regulated at the federal level. This regulation of MHDV fuel efficiency was mandated by Congress in 2007 in the Energy Independence and Security Act of 2007 (EISA), P.L. 110-140. Section 101 of EISA required the U.S. Department of Transportation (DOT) to promulgate fuel efficiency standards for MHDVs. The statute anticipates that these standards would be updated over time and requires DOT to provide 4 years of lead time between promulgation and enforcement of fuel efficiency standards, and also requires a period of 3 years of stability once the standards are in effect. On May 21, 2010, President Barack Obama directed the National Highway Traffic Safety Administration (NHTSA), on behalf of DOT, to issue MHDV fuel economy standards in close coordination with GHG emission standards to be promulgated for the same vehicles by the U.S. Environmental Protection Agency (EPA). Section 108 of EISA also requires the secretary of transportation to contract with the National Academy of Sciences to undertake a study on the technologies and costs for improving fuel efficiency in MHDVs. Upon completion of the National Research Council (NRC) Phase One Report² and its own study of fuel efficiency standards for MHDVs, DOT was instructed to promulgate by rulemaking a fuel efficiency program for MHDVs that is “designed to achieve the maximum feasible improvement” in fuel economy and to “adopt and implement appropriate test methods, measurement metrics, fuel economy standards, and compliance and enforcement protocols that are appropriate, cost-effective, and technologically feasible for commercial medium- and heavy-duty on-highway vehicles and work trucks” (49 U.S.C. § 32902(k)(2)). Figure S-1 provides a schematic of the different phases of the regulation, the model year (MY) vehicles included, and the types of MHDVs that are considered in the standards. (Figure presented) The committee issued its Phase Two First Report in 2014.³ The Summary chapter from the 2014 report is provided as Appendix E for reference. It provided guidance for the Phase II rule, which was directed at the post-2018 time frame. The present report is the final report of the NRC Phase Two study and covers a broader range of technologies and issues focused on the 2022 to 2030 time frame. The Summary captures the overarching report themes of vehicle technology progress, alternative technology and approaches, economic assessment and considerations, future regulatory framework, and interim evaluation. Given the broad scope of the statement of task (SOT), it focuses only on the most significant recommendations. The associated findings are addressed in the individual chapters. Chapter 1 contains a detailed mapping of the SOT items to the report chapters, but briefly, the report is organized with a discussion of MHDV regulation structure and compliance (Chapters 2 and 3), followed by powertrain (Chapter 4) and power demand reduction (Chapter 5) technologies and fuel consumption benefits (Chapter 6), hybrid and electric and battery technologies (Chapters 6–8), freight efficiency (Chapter 9), intelligent transportation systems and automation (Chapter 10), and manufacturing, cost-benefit, and alternative regulatory approaches (Chapters 11–13). A number of appendixes follow with detailed information.