A key strategy to improving the real-world fuel consumption and emissions of medium and heavy duty vehicles is the hybridization of these applications. Unlike the passenger vehicle market, medium and heavy duty applications are typically comprised of a range of components from a variety of manufacturers. The vocational market diversity and size places considerable demand on fuel efficiency and emission compliance. Medium and heavy duty applications have the ability to be successfully hybridized in ways that are not currently, or would not be practical within a passenger vehicle. This would also drive greater truck and bus vertical integration of the hybrid components. However, medium and heavy duty manufacturers have been prevented from certifying a full vehicle level platform due to the current engine only certification requirements. This lack of a vehicle level certification procedure limits the ability to optimally integrate conventional and hybrid platforms and limits the applicability of certification results to real-world improvements in fuel economy and emissions.To determine future opportunities and direction, a joint study was conducted by Navistar and Argonne National Laboratory (ANL) to understand how these technologies may be practically integrated into medium and heavy duty applications. This study included the evaluation of real world vehicle data, current certification, regulatory processes and marketing data. From this, many challenges and opportunities were discovered. This paper identifies the issues and opportunities facing medium and heavy duty hybridization. The context includes the current market characteristics, past and present lessons learned from light duty passenger vehicle hybridization and regulatory barriers for hybridization. This also includes the diversity of hybridization options within the medium and heavy duty market and the proposed opportunities for widespread adoption.